Seoul — South Korean President Chun Doo Hwan and Japanese Prime Minister Yasuhiro Nakasone have opened a new chapter in the long, often unhappy relationship between their two neighboring countries.
For the two days of Mr. and Mrs. Nakasone's visit to Seoul (Jan. 11 and 12), the rising sun flag has fluttered companionably with South Korea's yin-yang flag from lampposts along the avenue of honor leading to the capitol. As Mr. Chun reminded his guests, this is the first time the two flags have been entwined since South Korea regained independence from Japan after World War II.
Mr. Chun and Mr. Nakasone agreed, a joint communique said, that ''it would be in the interest of both the South Korean and Japanese peoples for the two countries, as neighbors pursuing the common ideals of freedom and democracy, to mutually maintain and develop close cooperative relations.''
This does not quite meet South Korea's desire for a more explicit trilateral partnership among Japan, South Korea, and the United States. But there is no question that Mr. Nakasone's visit, the first substantive visit by a Japanese chief of government to South Korea, has vastly improved the prickly relationship between Tokyo and Seoul. This in turn accords with American strategic interests, since the United States is the security guarantor both of South Korea and of Japan.
A longstanding dispute over economic aid was also settled. Japanese sources said the two sides had agreed on a total figure of $4 billion that Japan would supply as aid to South Korea during the fifth five-year plan, which began in 1982. Government aid will be $1.85 billion at concessional interest rates, while an additional $2.15 billion will take the form of export-import bank loans, at higher interest rates subject to guidelines of the Paris-based Organization for Economic Cooperation and Development.
The communique did not tie Japanese aid either to the issue of Korean security or to the heavy imbalance in Japan's trade with South Korea - two issues that had earlier been cited by South Koreans as justification for their aid demand.
In fact, President Chun seemed to have gone out of his way to make Mr. Nakasone's visit as pleasant as possible, one in which there would be a meeting of hearts rather than an attempt to score points off each other.
Mr. Nakasone, for his part, paid homage to the determination with which the South Koreans are facing up to North Korea's military threat while building up their own economy. He firmly ruled out any military cooperation on Japan's part, saying that Japan's no-war Constitution rules out the right of collective self-defense. At the same time he said Japan was conscious of its own responsibilities as a member of the free world and would supply all help within its means, except military.
The Japanese prime minister repeatedly stated that he and his people ''took to heart'' Japan's past unhappy record in Korea, though he did not explicitly refer to the history of Japanese invasions of Korea or the 36-year period of Japanese colonial rule.
Both he and Mr. Chun emphasized the start of a new era in Korean-Japanese relations, featuring wider, deeper political, economic, and cultural ties. ''It is utterly wrong that two countries which are the closest geographical neighbors should be so far apart,'' Mr.Nakasone said.
Neither Mr. Nakasone nor Mr. Chun mentioned one factor that considerably eased tensions between the two countries, making Mr. Nakasone's visit possible. That was General Chun's release last month of his principal political rival, Kim Dae Jung. Mr. Kim was given a medical discharge from his sentence of life imprisonment and exiled to the United States with his family. The move is believed to reflect a greater sense of political confidence on General Chun's part. He remains committed to his pledge to step down in 1988 after one seven-year term as president.
The two leaders obviously enjoyed each other's company. After the official banquet Jan. 11, Mr. Chun invited Mr. Nakasone to a totally informal, sports-jacket occasion omitting all mention of policies. The two leaders and their interpreters took turns singing songs to each other, a form of entertainment popular in Japan and Korea. Mr. Nakasone's rendition of ''Yellow Shirt,'' a popular Korean song, apparently brought the house down.