Prime Minister Yasuhiro Nakasone today proposed increasing Japan's defense spending by 6.9 percent to almost $13 billion in the coming fiscal year. It would be the largest increase in real terms in at least five years.

A draft national budget adopted today by his Cabinet would continue recent years' steady expansion of the Japanese military establishment while holding most other programs steady or cutting them.

Some analysts believe that the plan could push outlays for the armed services above 1 percent of the gross national product. That level, adopted as an official ceiling in 1976, has major psychological importance here.

Adjusted for inflation, Nakasone's proposed increase would be 5.4 percent. That compares to 3.9 percent real growth in 1981, 4.6 percent in 1982, 4.3 percent in 1983 and 4.8 percent in the current year.

Foreign aid spending would rise by about 10 percent to $2.4 billion, with about $245 million for Africa. Japanese officials say that would put them close to their objective of doubling overseas aid for the five-year period ending in fiscal 1985.

Officials here are citing the budget as proof of Japan's determination to broaden its commitment to the western alliance and to the developing world.

Nakasone is scheduled to meet with President Reagan in Los Angeles next week. Japanese officials hope that the arms budget increase will offset growing dissatisfaction in Washington over the mounting trade deficit with Japan.

But in a meeting with American journalists this week, Nakasone played down the international justification. "Defense is something that we engage in for the sake of Japan herself," he said. He acknowledged that the budget may provoke domestic controversy.

The budget emerged from long negotiations between competing ministries and within the ruling Liberal Democratic Party. It now goes to the Diet, or national legislature, which in past sessions has approved military budgets with little change.

Japan's armed services are called "self-defense forces" because the postwar constitution bans maintaining a war-making potential. They have only about 180,000 people in uniform but in spending rank as the world's eighth largest.

Under the budget, 1985 equipment purchases would include an unspecified number of ground-to-air Patriot missiles, 14 F15 jet fighters, 10 P3C Orion antisubmarine patrol planes and three destroyers.

Other money would go toward improving ability to fight an extended war -- in which Japan's capability is low. About $600 million would be used for ammunition, a 28 percent rise over last year.

About $330 million would go for housing, work facilities and support services for the 48,000 U.S. troops stationed in Japan.

Nakasone compiled his budget amid an inconclusive debate over the 1 percent ceiling. Some members of the ruling Liberal Democratic Party, apparently including Nakasone, favor dropping the ceiling on the grounds it is an arbitrary limit that will prevent Japan from meeting its defense commitments.

But there is a pacifist streak in Japanese politics. An official break with the ceiling could make Nakasone vulnerable to critics in his party and the opposition.

As proposed, the military budget equals 0.997 percent of the projected GNP for fiscal 1985. But it provides for only a 1 percent pay raise for military personnel. Next August, a general pay rise for government employes could give soldiers a bigger raise, pushing defense spending over the 1 percent ceiling.

In addition to raising spending, Nakasone has fostered closer ties in the field with the U.S. military. Earlier this month, Japan and the United States signed a military cooperation agreement. Its contents were secret, but Japanese newspapers said it included operational details for repelling a Soviet invasion of Japan.

The total draft budget calls for $215 billion of government spending, a 3.7 percent increase over the current year, with a $48 billion deficit.

Public works outlays would go down 2 percent, small business programs 6 percent and food price supports 15 percent.