Japanese policies designed to further open the nation's markets and reduce its enormous trade surpluses will continue with little change regardless of who succeeds Yasuhiro Nakasone as prime minister, according to a leading contender for the job.

"There seems to be no alternative but to carry on," Finance Minister Kiichi Miyazawa said in an interview in Washington Saturday. The Japanese public has supported Nakasone, he asserted, and recognizes that his initiatives are needed to safeguard relations with the outside world, particularly the United States.

Miyazawa said that Nakasone's market-opening efforts, including a two-year-old "action program" of tariff cuts and regulatory reform, are already having the desired effect of reducing Japan's surpluses, although slowly.

In 1986, the surplus with the United States totaled almost $60 billion, a figure so large that it was inconceivable only a few years ago. Many U.S. policymakers have suggested that Japan's reforms are moving much too slowly, but Miyazawa stressed several times that the important thing is that a trend toward reducing the surpluses has been set.

Once a career official in the ministry he now heads, Miyazawa is one of three "new leaders" in Nakasone's ruling Liberal Democratic Party who are competing for the prime ministership. The others are party secretary general Noboru Takeshita and former foreign minister Shintaro Abe.

Nakasone is scheduled to step down at the end of October after five years in office. Even if his general policies are continued, many analysts here and in Japan question whether his successor will implement them with the same vigor that Nakasone has shown.

In an hour-long conversation, Miyazawa repeatedly praised Nakasone. He said his policies were working and that Japan intended to "play the game by the same rules" as its trading partners.

He conceded that change was coming slowly. "Since it involves some traditional customs," he said, such as low-level officials in the Japanese trade bureaucracy refusing to relinquish the power they enjoy, "it may take some more time."

He cautioned that regardless of progress in negotiations, the relationship with the United States will never be completely calm. "If one thing is solved, another thing crops up. ... That we must learn to live with.