Japan is drafting a $100 billion, five-year military spending plan that would expand sea lane and air defense capabilities and probably mean the formal end of a policy of holding defense spending to below one percent of gross national product, according to press reports here.

The plan, now being circulated within Japan's Defense Agency, is expected to be adopted officially as a government target sometime this summer. The Diet, or national legislature, then would have to approve money for it year by year.

The plan would continue Japan's steady expansion of military spending in recent years but bring no radical changes in the speed or direction.

It is part of a long-term effort to take over some of the regional defense burden now shouldered by the United States, which maintains about 50,000 troops in Japan.

Here are the main points of the plan, according to the Asahi Shimbun newspaper:

* U.S.-designed P3C Orion antisubmarine patrol planes, now numbering about 50, would rise to 100. Combat ships would increase from 49 to 63. Submarines would rise by five to reach a level of 15 or 16.

In the early 1980s, Japan agreed to work toward building a capability to defend its sea lanes up to 1,000 miles from its shores. Thus, much of the new equipment is directed at that goal.

* Japan's Air Force, by the end of the plan, will have 190 F15 fighters, 65 more than it currently has. The planes are manufactured in Japan under license from McDonnell Douglas. The old Nike surface-to-air missiles would be replaced with the new Patriot missile system.

* Japan would buy four more Grumman E2Cs, a small, propeller-driven radar plane, bringing its total to 12 of the aircraft.

* Army divisions on the northern island of Hokkaido, Japan's closest point to the Soviet Union and the traditional focus of ground defense, would be reorganized. The Army would get 40 new antitank helicopters.

The new plan would come into effect with the fiscal year that begins April 1, 1986. The current fiscal year's budget already has Japan butting up against the one percent of GNP ceiling on defense spending that was adopted by the Japanese Cabinet in 1976.

The government of Prime Minister Yasuhiro Nakasone wants to scrap the ceiling. But lingering antimilitary sentiments dating from 1945 make it politically difficult.

However, it is widely expected to be broken formally this summer for the first time since it went into effect, when the Diet probably will approve a pay raise for all government employes. Larger paychecks for the 180,000 people in uniform automatically would take spending above one percent of GNP.

The Japanese government is predicting an average 4 percent per year GNP rise for the five years covered by the plan. The military plan would then exceed one percent unless economic growth were considerably higher.