Japan's new prime minister, Zenko Suzuki, declared today that relations with the United States remain the cornerstone of his nation's foreign policy and pledged his best efforts to meet U.S. expectations in the military and economic fields.

The surprise designation of the 69-year-old domestic politician 10 days ago left his constituents and the rest of the world in the dark about the international views of the new Japanese leader, who has virtually no record on foreign policy issues.

Suzuki began sketching in his ideas in press conferences of the past few days, and he advanced that process today in a 50-minute interview with Washington Post correspondent William Chapman and me. It was his first interview with foreign journalists since taking office and his most extended comment to date on foreign affairs.

The new Japanese leader repeatedly emphasized his desire for continuity with the policies of his predecessor and longtime associate, Masayoshi Ohira, whose sudden death of heart failure last month set off a scramble for succession.

Suzuki was picked essentially because he is a consensus-building politician within the ruling Liberal Democratic Party. Today he spoke of his desire to utilize his hallmark technique of harmony through dialogue in the troubled international arena.

In keeping with Japanese tradition and practice, the new leader avoided specific commitments in discussing the plans for his administration but gave clear indication in general terms of the directions he intends to pursue. Suzuki said he intends to "respect fully" a pledge by Ohira to President Carter last May to "seriously consider" stepping up Japan's military efforts, which now consume less than one percent of this country's vast annual output.

Prodded by U.S. Ambassador Mike Mansfield and a series of public statements from Washington, Japanese officials are now considering a speedup in previously scheduled defense programs.

The new foreign minister, Masayoshi Ito, said in a separate interview that the amount of the Japanese effort is still being debated by contending government agencies, but he predicted the result will be a military spending increase.

"We would like to do as much as possible with our ally, the United States, in the area of security," said Suzuki. He added that the efforts would be made within the framework of Japan's postwar policy of not acquiring offensive military power or nuclear weapons.

Suzuki also said that Japan's contribution to peace and security in the world should not be confined to "the narrow area of defense or military affairs" but should include a further increase in economic aid and technical assistance to developing countries "to preclude any conflicts before such conflicts happen."

In the economic field, Suzuki forecast additional efforts to solve conflicts over automobile exports by encouraging Japanese automobile company investments in manufacturng facilities in the United States. He said Toyota and Honda, two of Japan's largest automakers, are studying such investments "in a very forward looking manner," language which is often used here to indicate probable approval.

"I think there is an element of brightness in our outlook" on the auto question, Suzuki said. He said Japan expects that the U.S. automobile industry will help to solve the basic problem by meeting the increased demand of American consumers for small, fuel-efficient cars. He said that in the meantime "Japan should not take advantage of the situation to exercise disorderly export tactics."

In general, Suzuki said, Japanese relations with the United States are "excellent." He added, "I think our relations have gone into a period of mature relations, and I firmly believe that this must be maintained and developed even further."

The new Japanese leader, who has been a member of parliament continuously since the postwar constitution was adopted in 1947, conceded that he has been only tangentially involved in diplomatic matters during his domestically oriented career as a consensus broker among competing interests and political factions.

"I'm sure my name is very little known abroad," he said with a grin.

Despite his relative unfamiliarity with international affairs, he answered questions directly, without hesitation, and without reference to a sheaf of notes provided by his staff.

Because anything Suzuki says on diplomacy at this early point of his administration tends to make policy as well as news, three aides took copious notes while he was being interviewed and a tape was made for the Foreign Ministry.Japanese reporters milled outside the interview room at the official residence and were briefed afterward by senior officials on his statements.

Suzuki, whose slogan for his new administration is "the politics of harmony," went out of his way to say at several points that he hopes to rely on extensive dialogue with international leaders as well as with domestic political parties and interests to smooth his country's way in the world of the 1980s. While he said no decision has yet been made, he hinted that he is thinking of overseas trips later this year, perhaps to Southeast Asia.

A trip to the United States will probably have to wait for 1981, he said, because American leaders will be busy this year with the elections.