Why the Japanese trade gap?

Japan's trade surplus with the United States and Europe stems from a ''gap in competitiveness,'' and not from Japanese trade barriers, according to a high-ranking Japanese trade official.

''The Japanese market is an open one, and we would like to see the businessmen of America and Europe make further marketing efforts and sales efforts in Japan . . . we would like to provide the industrial and technical cooperation that is required,'' said Tadayoshi Nakagawa, director general of the International Trade Policy Bureau of the Ministry of International Trade and Industry.

Unusually high unemployment and consistently high trade deficits have sparked anger and moves toward protectionism in the United States and the European Community, he said.

Japan's trade surplus with the US - projected at between $18 billion and $19 billion in 1982 - and a US unemployment rate in excess of 10 percent have created the impression among Americans that ''Japan exports unemployment,'' Mr. Nakagawa noted.

But in Washington on Nov. 10, Deputy US Trade Representative David R. Macdonald said America's patience is running out because, he charged, Japan has failed to live up to promises to open its markets to US products.

''Our flexibility is about gone in dealing with the Japanese,'' Mr. Macdonald said.