Tokyo — Prime Minister Yasuhiro Nakasone has decided to increase Japan's defense spending by 6.55 percent at a time when every other budget item except foreign aid has been held to ceilings close to or even below last year's figures.
The increase, which brings the 1984 defense budget to 2.934 trillion yen ($12 .5 billion), is bound to disappoint Washington. But it represents the maximum that Mr. Nakasone feels is politically possible at a time when Japan's cumulative national budget deficit has reached 120 trillion yen ($515 billion).
''I think this will help to improve our international relations,'' Mr. Nakasone said in an oblique reference to Washington. ''But the fundamental point is that we must be willing to depend on ourselves for our own defense.''
Mr. Nakasone is sending Foreign Minister Shintaro Abe to Washington today. One of Mr. Abe's chief tasks will be to explain that the 6.55 percent increase represents an act of considerable political courage.
Mr. Abe will not have an easy time. Japan has pledged repeatedly to increase defense spending to a point where it can defend its sea lanes out to a distance of 1,000 miles from its shores. To this end, it is in the midst of a five-year buildup aimed principally at acquiring 14 new escort destroyers, 75 F-15 Eagle fighters, and 50 P-3C Orion antisubmarine patrol aircraft.
Even this is not enough to satisfy Washington, which, according to some reports, would like Japan itself to have 125 P-3Cs. The new budget allows the Defense Agency to buy eight P-3Cs this year, as it requested, and 17 F-15s - four fewer than it wanted. This is the second year of the five-year defense plan , at the end of which theoretically the buildup should be 40 percent complete. But, including all the other items on the list, Defense Agency officials calculate they will have reached less than 30 percent of the five-year target by 1985.
Most political observers here believe that the five-year buildup will have to be stretched out if the target is to be reached. Yet Washington, in the past, has pressed Japan to accelerate, not to delay, the pace of the buildup.
From the standpoint of Japanese politics, however, it is nearly impossible to justify larger defense increases at a time when social welfare spending is being allowed to increase by only 2 percent and public works expenditures will actually be reduced by 2 percent. Only foreign aid is being increased by a conspicuous 9.7 percent - and this, too, has more to do with Japan's need to be seen as a responsible member of the world community than with any attempt to gain popularity with the voters.
The 6.55 percent increase for the defense budget is 0.05 percent higher than last year's 6.5 percent figure.
''Please be sure you report the figure as 6.55 percent - not 6.5 or 6.6,'' said a defense official briefing foreign journalists. ''That 0.05 percent is very important.''
Among other things, it brings Japan's defense spending to 0.99 percent of gross national product.
To most Western countries, 1 percent of GNP is a ridiculously low figure to be spending on defense. (The US defense budget was 6.5 percent of GNP in 1983.) But considering the strength of pacifism in postwar Japan and the fear of Japanese military resurgence on the part of Asian neighbors invaded by the island empire during World War II, successive Japanese Cabinets since the 1960s have kept defense spending at or below this figure.
If Mr. Nakasone had won the Dec. 18 general election last year, he might have been able to take a bolder line on defense. But his Liberal Democrats lost 36 seats in that election, and depend on a coalition with a minor party to maintain a stable majority.
If economic conditions in Japan improve, as they have been doing gradually, and GNP grows this year by more than 4 percent, there will be a larger share of the pie for defense next year.
Meanwhile, Washington's dilemma is that, much as it would like to press for greater defense efforts by Japan, it is not going to find any politician here friendlier to its own global strategy than Mr. Nakasone, or more willing to look for opportunities to increase defense spending..