The Japanese government decided tonight to increase its defense budget by about 6.6 percent in the year beginning April 1. Spending for most other government programs will be held at current levels.

The planned military budget, 3.344 trillion yen or about $16.7 billion at current exchange rates, will keep Japan generally to the schedule of the five-year buildup program it adopted in September, according to Defense Director General Koichi Kato.

About a quarter of the new budget, which covers Japan's fiscal 1986, will go toward purchase of new equipment, including down payments on 12 new F15 fighters, 10 P36 antisubmarine planes and three destroyers.

The spending figure was reached after a day of intense negotiating among Cabinet officials and leaders of the ruling Liberal Democratic Party. Technically, it is only a draft, but final passage without significant change by the Diet, or parliament, is seen as assured because of ruling party control there.

The growth will be roughly on a par with the current fiscal year's. It will continue an expansion in Japan's 245,000-member "Self-Defense Forces" that has been under way with few interruptions since they were established in 1954.

The United States, which maintains 48,000 troops here, has pushed Japan for years to shoulder more of the cost of regional defense. Many U.S. analysts say that its current growth rates are not adequate to meet the operational objectives Japan has set for itself, such as the patrolling of sea lanes 1,000 miles out from its shores.

However, some U.S. officials suggest that pacifist sentiments inside the country and suspicions among neighbors of a revival of World War II-style militarism make a significantly faster expansion politically unfeasible. There also is a an element of opposition to faster increases within the ruling party. Many of its members believe low government spending has been a factor in the country's prosperity.

Japan is the United States' most important ally in the region. In general, U.S.-Japan military relations are rated by both sides as extremely cordial at present, apparently unaffected by the serious tension hampering trade relations.

Adjusted for inflation, the new arms budget represents a real increase of about 5.4 percent, Japanese officials said. That would be one of the faster rates of the 1980s and almost double the 3 percent real growth rate of U.S. military spending sought by President Reagan for 1986.

But Japan's outlays as a portion of its total economy, now the world's third largest, remain far below those of the United States, which spends about 7 percent of its gross national product on defense.

Officially, Japan's are just under 1 percent of its GNP, a ceiling established by a 1976 Cabinet order. However, Japan does not count pensions for World War II veterans and other indirect outlays that are included in the calculations of NATO countries. When these are considered, Japan's figure rises to about 1.6 percent.

The new budget is unofficially calculated to be equivalent to about 0.993 percent of GNP expected for the fiscal year. However, it does not include a 1 percent pay increase normally built in at this stage. Armed forces personnel normally get a raise during the year, and anything over 1.7 percent probably would result in the level being exceeded.

Routine pay raises were expected to break the 1 percent barrier this year. However, the GNP estimate was marked up later during a review, and this year's spending now is expected to remain below the ceiling.

In addition to down payments on new equipment, the budget provides for funding commitments in later years to rise by about 7.2 percent. It also provides for 600 new personnel.

Defense Agency officials said that precise figures on how much Japan would contribute to the upkeep of U.S. forces in Japan were not available. But a so-called "sympathy fund" that pays for such things as housing construction and some wages of Japanese employes would rise by about 9 percent to reach about $450 million. Total Japanese spending for U.S. facilities will be roughly three times that sum.

The Japanese normally cover about a third of the total cost. Newspapers here have reported that the United States has pushed for more because the recent 20 percent devaluation of the dollar against the yen has cut into the buying power of its budgets here.