Japanese Prime Minister Zenko Suzuki, in White House talks with President Reagan, moved closer yesterday to accepting Pacific defense roles involving major expansion of Japan's military power.

In a joint communique marking the end of two days of unusually cordial talks, Suzuki pledged that Japan will attempt "even greater efforts" to improve its defense capabilities "in Japanese territories and in its surrounding sea and air space."

Suzuki also agareed, in a vaguely worded sentence that may open the way for months of detailed negotiations, on "the desirability of an appropriate division of roles between Japan and the United States" in the Far East.

During the White House session, Reagan said the United States seeks a division of Asian roles on the basis of equal partnership with Japan, according to a White House briefing.

U.S. officials made clear in the last several days that they would like to see Japan assume the responsibility for air and sea patrol of a large part of the Western Pacific to relieve U.S. forces for action elsewhere, and that they intend to discuss this and other concrete proposals in a mid-June meeting of U.S. and Japanese military experts.

Suzuki, in talks with Reagan and public comments, continued to reject all-out remilitarization as impossible because of public opinion and Japan's "peace constitution."

If Japan ever decides to rearm fully, Suzuki said in a meeting with Washington Post editors yesterday, it could become a world military power third only to the United States and Soviet Union , in view of its economic, scientific and technological capabilities.

At this stage, Suzuki continued to espouse the policy of "minimum necessary defense" employed for "purely defense purposes," he told The Post.

But, in the meeting with Post editors and in remarks at a National Press Club luncheon, Suzuki expresses the aim of amassing enough military power to patrol the seas for a radius of several hundred miles from Japan, and to patrol sea lines of communications, the routes of the oil supertankers that are Japan's energy lifeline, for about 1,000 miles from home.

These geographical limits of Japan's existing defense authority have been previously stated to the Diet, or parliament, by government officials in Toyko. But Suzuki's repetition of them here was in the context of goals rather than theoretical outside limits, suggesting the necessity of a buildup of men and equipment to meet them.

How to go about an improvement of military equipment and capability in coordination with the United States within the framework of Japanese law and public opinion "in the most important task facing us today," Suzuki told Post editors.

The immediate and practical problem may involve finances more than anything else. Suzuki made it clear that he plans no increase in taxes this year after the largest-ever postwar tax increase last year and that he is seeking to reduce deficit financing.

He told Reagan, according to a White House briefing, that "sacrificing" everything else in favor of military programs would cause grave political difficulties.

Reagan responded in "equal frankness" by referring to large-scale Soviet military efforts and declaring that "freedom may be in danger . . . in all parts of the world," according to the White House briefing.

While adding that he "understood" Japan's military constraints and that "it will not be our policy to pressure Japan," Reagan's words constituted strong persuasion in the Japanese-American context.

Administration strategy is to forge an agreement in general terms with Suzuki on the threat from Soviet power throughout the world, especially in Asia, and then to work within this framework for increases in Japanese military strength to respond to the common problem.

The two-day meeting here ended with some apparent success on the general points, including a communique that ranged over global strategic issues much more widely than previous Japanese-American documents of this sort.

The final Reagan-Suzuki meeting yesterday morning was scheduled for 30 minutes, but lasted three times that long and concentrated mostly on the defense issue.

Saying goodbye to his visitor outside the Oval Office, Reagan said, "We have discovered we're in agreement on a number of broad issues."

Suzuki, in turn, said the two leaders had achieved "a basic convergence of views and perceptions about the important matters facing the world today." He called Japan and the United States "true partners and true friends."

On an item of persistent contention during the Carter administration, Reagan apparently agreed to ease U.S. restrictions on the reprocessing of nuclear fuel in Japan.

"The president endorsed the view of the prime minister that reprocessing is of particular importance to Japan," said the joint communique, adding that the two leaders agreed on prompt consultations to work out "a permanent solution" to pending nuclear reprocessing issues.

The Carter administration strongly opposed unrestricted nuclear fuel reprocessing, an operation that produces plutonium, the raw material of nuclear weapons. Judging by yesterday's communique and earlier indications, the Reagan administratin is much less concerned.

The early removal of restrictions imposed by the Carter administration on reprocessing of U.S.-supplied nuclear fuel at Japan's Tokai pilot plant is necessary for its operation after this summer.

The joint communique also mentioned construction of an additional nuclear reprocessing plant in Japan. A White House briefer said the Reagan administration intends to "work out the details" to permit these questions to go forward.

Suzuki is to meet U.S. Cabinet members in the economic field this morning before leaving Washington for Ottawa. Due to the voluntary automobile export curbs that Suzuki imposed on the eve of his journey to Washington, economic strains played a relatively minor in his talks with Reagan.