The Japanese Cabinet this afternoon adopted a $76 billion, five-year defense spending plan that could break a longstanding official policy holding military budgets to 1 percent of gross national product.

Based on current projections of GNP by the government's Economic Planning Agency, the plan would mean yearly defense spending averaging 1.04 percent of GNP.

Despite these figures, Chief Cabinet Secretary Takao Fujinami told reporters that the government will continue "to make efforts to respect the intent" of the 1 percent ceiling, which was adopted in 1976.

Officials here fended off questions about the seeming contradiction between the five-year plan and Fujinami's statement. However, the question could become academic if Japan's economy were to grow faster than expected, so that actual spending would remain below 1 percent.

The plan is not expected to change the basic thrust of the ongoing build-up of Japan's 245,000-member armed forces. But it will be important if it does end the psychologically crucial ceiling.

Earlier this month, elders of the ruling Liberal Democratic Party turned down a request from Prime Minister Yasuhiro Nakasone to scrap the spending ceiling formally. But as a compromise, they approved creation of the five-year Cabinet spending plan.

Despite Japan's economic success, its government is cash-short and running a deficit proportionately larger than that of the United States. About 20 percent of Japan's national budget is being financed through borrowing, making larger defense spending a financial as well as a political issue.

Today's action followed all-night bargaining within the government. The Defense Agency, which is in charge of military affairs, had pressed for spending equivalent to about $80 billion but backed down in the face of opposition from the Finance Ministry and parts of the ruling party.

Fujinami pledged that Japan would maintain a "strictly defensive" posture and would not become a military power.

The government also announced today that it intends to increase foreign economic assistance in stages from an estimated $4 billion in 1985 to about $8 billion in 1992. This disclosure was aimed at overseas expressions of concern about a rebirth of Japanese militarism.

One of the defense plan's few new elements is to increase Japan's F15 fighter force to 187 aircraft, up from the previous target of 150. Japan now has about 50 of the U.S.-designed aircraft, which are assembled under license here.

The plan also would increase the number of P3C antisubmarine planes from 16 to 100 and submarines from 14 to 16. It also would largely complete the replacement of obsolescent Nike air defense missiles by modern Patriot missiles.