Japan moves to center stage. After five years as prime minister, the charismatic `king' of Japanese politics is about to retire. Yasuhiro Nakasone leaves behind a changed country with a new international outlook.

SOME years ago, Yasuhiro Nakasone reminisced to a visiting journalist that as a child his playmates always chose him to be their ``king of the mountain.'' The ``king'' is about to step down. In a few days, the charismatic politician whose beetle brows delighted cartoonists will hand over the gloomy, cave-like prime minister's residence to his designated successor, diminutive Noboru Takeshita, and return to private life.

It will be an active retirement: His booming baritone will still echo occasionally across the Diet chamber, and behind the scenes he will probably carve out for himself a new role as kingmaker.

Meanwhile, in Washington and London, Moscow and Peking, government and media analysts strive to sum up the meaning of Mr. Nakasone's five years as premier and the changes they have wrought.

First, there is wide agreement that Nakasone brought his hitherto isolated island country onto the center of the world stage.

``He has been outstanding'' in so doing, says Mike Mansfield, United States ambassador Ambassador to Japan. ``He has identified himself solidly with the West.'' In the process, he formed ``an intensely personal relationship with President Reagan.''

Second, Nakasone brought a new style to the prime ministership of Japan. Some have called it ``presidential''; others are content to characterize it as ``activist.'' Be that as it may, Nakasone changed Western perceptions about the role of a Japanese prime minister.

``The next prime minister of Japan,'' said one State Department official, ``is going to have to be able to trade jokes - in English - with his fellow heads of government. He is going to have to speak out, not just about cars and semiconductors, but about SS-20s and the defense of the free world. And his logic will have to be clear and comprehensible to Western minds.''

Third, Nakasone's work is incomplete. ``His role was to point out the way we should go, and then take us a few steps down that road,'' said an adviser in the ruling Liberal Democratic Party. He established ``a consensus within the party that we have to walk the path of internationalization.''

But internationalization becomes progressively more painful and difficult as it bites into the core of how Japanese society is organized.

``It isn't just learning to speak idiomatic English or sipping your soup without slurping,'' said a Japanese with decades of experience living abroad. ``It's changing our own institutions, our own way of doing things. And that's not easy.''

Nakasone was the first Japanese prime minister to use the annual summit of the seven leading Western nations as a vehicle to assert his country's presence and responsibility within the Western alliance and more broadly within the global community. He did this, first of all, by speaking out. Most of his predecessors were either silent or used such tortuous circumlocutions that no one knew what they were saying, or where they stood.

``I have no impression whatsoever of [former Premier Takeo] Miki or [Zenko] Suzuki,'' said a summit participant from Western Europe, now retired. ``As for [Masayoshi] Ohira, we could all see he was thinking very deeply about the subject at hand. But somehow his thoughts never took shape as words we could understand.''

Nakasone also softened, though he could not entirely erase, the image of Japan as an ``economic animal'' pursuing only its own selfish ends.

At the early summits, the Japanese shied away from security questions and wanted to concentrate only on economic issues. As a result, when French President Val'ery Giscard d'Estaing decided to host a wide-ranging discussion of East-West security issues at Guadeloupe in 1979, he left the Japanese out, even though the four summiteers who did participate (Mr. Giscard d'Estaing of France, Helmut Schmidt of West Germany, James Callaghan of Britain, and Jimmy Carter of the US) took up not only security in Europe but also China and the Persian Gulf - subjects of immediate concern to Japan.

Nakasone, at the first summit he attended (Williamsburg, 1983), broke Japan's self-imposed taboo and spoke up strongly on doing away with Soviet SS-20 missiles not only in Europe, but in Asia as well. It is perhaps fitting that as he ends his premiership, Washington and Moscow are preparing to do just that, the Kremlin having given up its insistence on keeping SS-20s in Asia even after withdrawing them from Europe.

In addition, Nakasone was very active at the dinners, the strolls, the small moments between formal summit sessions - joking with Mr. Reagan at one moment, beckoning to Britain's Margaret Thatcher in the next. These informal encounters, said a Japanese official accompanying Nakasone to the last summit (Venice, in June) are extremely important in setting the tone for the formal sessions.

THE ``Ron-Yasu'' relationship is well known, but Nakasone also established first-name relationships with most of the other Western leaders. None of his predecessors achieved so intimate a link with an American president or with other summit leaders.

These links, first and foremost the Ron-Yasu relationship, have benefited Japan enormously during a period when the country's astronomic trade surpluses created continuous friction with the US and other Western powers.

``If he [Nakasone] had not been in office during this particular period, the situation might well have become worse,'' says Ambassador Mansfield.

In domestic politics also, Nakasone brought a new style to the office of prime minister. Mr. Takeshita observes that at the very start of Nakasone's political career, Nakasone advocated choosing the prime minister by direct popular election, and that from the day he entered the Diet at the age of 28, he was determined someday to become prime minister himself.

``That's the difference between Nakasone and me,'' said Takeshita. ``I didn't really think of the prime ministership as a goal until years after I was elected to the Diet.''

Nakasone stood out all the more because Japan is a land of rule by consensus - whether in politics, business, or in daily social relationships. The prime minister's job is more like being the chairman of a committee than the president of a large corporation. The bureaucracy is ponderous and precedent-bound, and each ministry jealously guards its turf. Nakasone bypassed the bureaucracy whenever he could, often creating private commissions that reported directly to him.

The Maekawa Commission, headed by a former governor of the Bank of Japan, was typical. In 1986, it analyzed succinctly the whys and wherefores of obstacles to more open markets in Japan and recommended bold action to remove them. The commission's report generated intense media interest in Japan and overseas, but its specific recommendations continue to gather dust.

The record in other fields, such as educational reform, is similar.

Was Nakasone's presidential style unsuccessful, then?

Some critics contend that this ebullient politician was all style and no substance; that his words were grand but that he lacked follow-through.

He came to office saying he was determined to simplify and streamline government, reduce and eventually eliminate Japan's deficit, change the tax structure so as to benefit office workers, and carry out educational reforms.

HE did indeed take the government out of two important areas: railways and telecommunications. The national railway system was privatized and split into seven regional companies. The telecommunications monopoly was likewise privatized and new enterprises authorized to enter the field.

But the government deficit is nowhere near being resolved. Quite the contrary: It may worsen because of the new emphasis (demanded by Japan's trading partners) on encouraging imports by stimulating domestic demand.

Tax reform is being tackled, but Nakasone had a serious defeat earlier this year when his proposal for a sales tax had to be scuttled in the face of determined resistance, not only from the opposition parties, but from within the ruling party as well.

Critics recall that when he became prime minister in November 1982, Nakasone led a faction that was only the fourth in size among the Liberal Democrats' five major factions. He would never have made the premiership without the active support of the Tanaka faction, led by former Prime Minister Kakuei Tanaka. Until the latter had to quit politics after a stroke last February, Nakasone ruled only on the sufferance of this faction, which controlled one-third of the Liberal Democrats in the Diet.

Nakasone has spoken eloquently of Japan's need to open its doors wide both to goods and to services, and of the urgency of changing his country men's isolationist mentality. But when it comes to opening up Japan's domestic market, or to allowing foreign companies to participate in the building of the Kansai International Airport in Osaka Bay, or to admitting overseas firms into the lucrative telecommunications field, actual progress has been painfully slow.

There is, in sum, a lot of unfinished business that Nakasone will have to hand on to his successor.

And yet, when asked to give an overall evaluation of Nakasone's performance as prime minister of Japan, most analysts are positive.

True, Nakasone was fortunate. He succeeded the undistinguished and inarticulate Zenko Suzuki and burst onto the world stage with a flood of language that Americans, in particular, wanted to hear: an assertion of solidarity with the US and the West in the face of Soviet expansionism, a pledge to accept a greater share of responsibility for the world's less fortunate countries, a promise to fling open Japan's doors and break down the barriers that kept Japan's markets - and many aspects of its society - closed off from the outside world.

A Liberal Democratic Party adviser concedes that Nakasone has only partly succeeded in opening Japan's doors. But could someone else have done better? This adviser thinks that, given the intensity of the entrenched opposition to change, Nakasone did as well as could realistically be expected.

``He changed the climate of opinion within the party,'' this adviser says. ``He established a consensus that we have to walk the road of internationalization. The road keeps getting steeper and the obstacles more formidable, because the easy things have all been done. But we agree there is no alternative.

``That may turn out to have been Nakasone's greatest contribution to his country and his party.''

A Nakasone chronology 1982 Nov. 26: Nakasone elected Prime Minister by the Diet (Parliament). 1983 Jan. 11: Nakasone's first visit outside Japan is to South Korea to usher in what he calls a ``new and vital stage'' in ties. Jan. 18-19: Nakasone visits Washington, establishes a ``Ron-Yasu'' first-name relationship with President Reagan, calls Japan an ``unsinkable aircraft carrier.'' May 28-30: Nakasone attends the Williamsburg summit of seven leading Western nations and plays an active role in drafting a statement urging the USSR to give up SS-20 nuclear missiles in Asia and Europe. 1984 March 23-26: Nakasone visits China, offers $2.1 billion in low-interest loans. June 7-9: Nakasone attends London summit, says June 11 ``We should not think in terms of the Atlantic vs. the Pacific or Europe vs. Asia.'' Oct. 29: Nakasone reelected President of Liberal-Democrat Party, assuring himself of two more years as Prime Minister. 1986 Jan. 2: Nakasone meets Reagan in Los Angeles as trade figures show Japan's trade surplus with the US, $18 billion dollars when Nakasone took office, is now $35 billion. On return to Japan Nakasone orders new measures to open Japan's markets to imported goods. March 28: US Senate passes resolution (92-0) blasting Japan for ``unfair'' trade practices and calling for retaliation. April 9: Nakasone appeals on television for Japanese to ``buy more foreign goods.'' Aug. 15: Nakasone visits Yasukuni Shrine honoring Japan's war dead on 40th anniversary of end of World War II. China assails him for this act. 1986 Jan. 15-19: Eduard Shevardnadze visits Japan, the first Soviet foreign minister to do so in 11 years. Agreements on technical cooperation and cultural exchanges signed, but no progress made toward settling Japan's claim to four Soviet-held islands off Hokkaido. April 7: Maekawa Commission, appointed by Nakasone, issues report calling on Japan to undergo a ``historic transformation'' away from export-led growth to expansion by stimulating domestic demand. July 6: Liberal-Democrats win landslide victory securing 304 of 512 seats in lower house. Although his term as party president expires at end of October, he is given one year's extension. Sept. 22: Nakasone says the Japanese educational level is high because they are homogeneous race, whereas in the US ``blacks, Puerto Ricans, and Mexicans'' lower the educational level. Two days later he offers ``heartfelt apologies.'' 1987 Jan. 23: Cabinet decides to do away with 1-percent-of-GNP limit on defense spending after having compiled a budget in which defense spending will come to 1.004 percent of GNP. March 2: Unemployment reaches record 3 percent; 1.8 million jobs, mainly in smokestack industries, have been lost because of high yen. April 1: In fiscal l986 (April 1-March 31, 1987), Japan's trade surplus with US soared to 52 billion. Thus throughout Nakasone's premiership Japan's surplus with US continued to grow.