Japan's message to US: quality not quantity is key

''In 10 to 20 years,'' says Deputy Foreign Minister Moriyuki Motono, ''the harsh Japanese competition will be seen to have been a stimulus to Americans to reorient their thinking.''

These words sum up a viewpoint widespread among Japanese leaders - that the United States retains undiminished economic potential for the future, but currently seems to have lost its way.

''Americans,'' said Mr. Motono, ''should have more confidence in their future. I have always said, once Americans realize their shortcomings, you will see a nation revitalized.''

At present, however, the shortcomings loom large, both to Americans and Japanese. The Japanese analysis of where the US stands today, though screened by courtesy, is tough-minded, sometimes unflattering, often blunt.

America is described as a great nation which appears to have lost confidence in itself and tends to transfer the blame to others - especially Japan - for failings within the US system itself.

''Too much criticism directed against Japan,'' said Mr. Motono in an interview, ''tends to obscure the basic reorientation of policies needed in the US.''

''The problem,'' said an economic specialist, ''is not that American workers cannot do what Japanese workers do. They can. But they must regain what you call the Protestant work ethic.''

In largely Buddhist Japan, a land teeming with the energy of an educated people striving to excel, the ''Protestant work ethic'' is very much alive.

''In Japan,'' said computer scientist Kazuhiro Fuchi, ''we are still very eager to produce the best TV sets or automobiles. You in the US have already done that and are looking beyond it.''

This is a polite way of saying that Americans no longer produce the best TV sets, automobiles, and a range of other goods.

As late as 1955, Japan had no automobile industry. When its now famous Ministry of International Trade and Industry (MITI) ''targeted'' a car industry as essential for Japan, says high-ranking MITI official Toshihiko Tanabe, ''there was widespread doubt among Japanese that we could build cars at all.''

In 1960, he said, Japan turned out 160,000 passenger cars and an equal number of trucks and buses. Twenty-one years later, the 1981 output of Toyota, Nissan, Honda, and others was 11 million vehicles, of which 7 million were cars.

Almost all Japanese leaders interviewed in government, industry, and academia paid tribute to the generosity of the US toward Japan after World War II. One called it ''a unique partnership between victor and vanquished.''

They cited the flow of Japanese businessmen, scientists, and engineers invited to the US to study American management and production techniques, and a reverse flow of American specialists who came to Japan to teach.

But they also cite the tendency of many US industrial workers, at least in the Japanese view, to demand a constantly rising standard of living, while drifting away from quality production as a top goal.

''Mass production, mass marketing, mass consumption - this approach served the US economy well, so long as there was no external competition,'' said Jiro Tokuyama, Dean of the Nomura School of Advanced Management.

Japanese experts perceive dual forces at work in the US economy - a struggle to regain its footing, or competitive edge, in consumer fields, while on the far frontiers of advanced technology, the US, due partly to massive military and space research, soars ahead of Japan.

''Japan's share of world GNP (gross national product) is 10 percent, compared to 2.5 percent after World War II,'' said Prof. Michio Nagai of Jochi University in Tokyo. ''The US share is 23 percent, down from 50 percent after the war.''

Comparing the two populations - 120 million in Japan, 230 million in the US - output of goods and services per capita is roughly the same. But what about the future?

Much will depend, experts agree, on the degree of quality revival in the US. An example is the story of Japanese car exports to the US.

Before Tokyo limited its auto shipments to 1.68 million units yearly - called a ''voluntary'' restraint, but made under strong American pressure - the Japanese share of the total US market was 16 percent. Today, after two years of restraint, the Japanese share tops 20 percent.

Because of recession, US demand for new cars declined. Yet Japan's market share rose because many Americans opted for Japanese models.

The relative output - and therefore living standards - of Japan and the US also depends on which better perceives future consumer trends and adapts to meet them.

''True targeting,'' said a Japanese expert, ''is a nation's vision of the future.''

Will it be good for Japan if its chief industrial rival, the US, regains a competitive edge?

''Emphatically,'' says Dr. Tokuyama. ''As soon as the US regains confidence and capability, we will begin to drain emotion out of our dialogue.''