The Japanese government, groping for a way to satisfy U.S. requests for a military buildup, today set 1987 at a target date for completing a basic defense plan first outlines five years ago.

The decision would not project a major increase in Japanese military strength and appears to be primarily a talking point for Prime Minister Zenko Suzuki when he meets president Reagan next week in Washington.

Defense is expected to be the major issue on their agenda and it is likely that Reagan will seek a general commitment for more Japanese assistance in the defense of Northeast Asia.

But Suzuki already has ruled out any major new military initiatives and, according to reliable sources, has decided on the alternative of setting a target date for achieving a modest buildup planned in 1976.

The decision was made this morning at a meeting of the National Defense Council, Japan's top military policy organization.

Despite the modest goal announced today, many influential figures in the government were opposed even to the idea of fixing a 1987 target date. Michio Watanabe, the finance minister, insisted at today's meeting that the final decision on how to reach that goal will not be made until next year. Watanabe exerts great influence on public spending and last year was able to reverse plans for a major increase in defense spending.

The most significant practical effort will be a commitment by Japan to fill out its naval and air forces to the levels fixed in a national strategic plan in 1976.

It would mean buying more modern ships and planes for the purpose of air defense and antisubmarine warfare.

There is no indication that the idea of fixing a deadline for these improvements will satisfy the Reagan administration or some members of Congress who contend that Japan is a laggard in the defense field.

Unlike the Carter administration, Reagan defense officials have been rather vague so far in defining what they expect Japan to do.

Reagan's defense secretary, Caspar Weinberger, has told Congress he expects Japan to do considerably more in the way of defense. But the administration, says it prefers to talk more about Japan's defense role in this area than about specific numbers, costs or percentage.

In general comments coming from Washington, Japan has been urged recently to prepare to take on a bigger role in defending sea lanes and patrolling for Soviet submarines in waters near Japan.

The Carter administration was disappointed last summer when Japan approved only a routine budget increase for its defense forces after first having appeared to promise a more substantial contribution. Its reversal was due mainly to a budget-pinching Finance Ministry.

Today's decision would promise that by 1987 the fundamental defense plan adopted in 1976 would be fulfilled. The 1976 plan had set general targets for the numbers of troops, planes and ships, but it did not specify when those targets would be achieved.

For example, the 1976 plan called for air self-defense forces to maintain a force of about 430 aircraft. Today, five years after the plan was unveiled, it has only 310 of them. The decision today would in effect require the addition of about 120 planes by 1987.

It would also commit Japan to a force of 60 antisubmarine ships, eight more than it now has. The maritime forces would be required to add about 40 aircraft to bring its strength up to the 220 planes authorized.

The ground self-defense force would add about 25,000 to bring its strength up to the authorized 180,000.