A rare debate has broken out over Japan's defense policy amid public charges that the government is slowly moving into the field of offensive weapons.

The debate was sparked by Japan's recent purchase of eight F15 fighter planes from the United States - the first of 100 F15s that Tokyo is to accquire during the next eight years.

Unlike the F4 Phantom jets that Japan purchased in 1969, the new F15s will be equipped with bombsights and devices for midair refuelling that will give them a greater flying range.

Japan's constitution renounces war and declares that the country shall have no "war potential." Under U.S. pressure in the early 1950s, that was reinterpreted to permit defensive weapons.

The decision to equip the F15s with bombsights and refueling devices, however, has sparked criticism that these planes could be used for attacks on other countries.

Some defense officials have broadened the debate by asserting the Japan could accquire aircraft carriers, and one high ranking civilian official has suggested that even tactical nuclear weapons are permissible for self-protection.

Since World War II, Japan - the only country to have suffered a nuclear attack - has banned a possession of nuclear weapons.

Prominent military leaders are contending that there is no practical distinction between defensive and offensive weapons, a direct assault on Japan's postwar policy of possessing only defensive weapons.

That has provoked a counterclaim that the constitution is being violated and that the government is laying the groundwork for a future rearmament with offensive weapons.

This largely philosophical argument recently focused on Japan's purchase of eight F15 fighter planes from the United States, the first batch of 100 F15s that will be bought during the next eight years.

In a significant policy change, the government is ordering them equipped with bombsights and devices for midair refueling that will give them longer flying range. Critics contend that these features make them offensive weapons for attacks on other countries and point out that when Japan purchased F4 Phantom jets in 1969 they came without either bombing or refueling devices.

Gen. Hiroomi Kurisu, chairman of the military's joint staff council, joined the debate with a blunt appraisal that is extremely rare for an active duty officer in Japan.

In a much quoted magazine article, Kurisu said constitutional restraints do not mean Japan must "sit and wait for self destruction since Japan has the right of self-defense as an independent country. It is possible to attack the other side's bases as a minimum and unavoidable measure.

"Military history indicates," he wrote, "that ina any war only an offensive can bring victory. With defensive measures alone, it is impossible to cope effectively with attacks which are taken powerfully from outside our country's sphere of action."

The goverment of Prime Minister Takeo Fukuda has not endorsed the general's views but has expressed satisfaction that a national debate on defense policy is under way. Socialist Leader Masashi Ishibashi, charges the Kurisu violated the constitution and says he should be punished.

Ishibashi, the Socialist critic, said in an interview last week that the government is "twisting" the constitution and slowly building a new consensus that will provide for new weaponry. He believes the intention is to become a military power equal to such countries as Britain, France and West Germany.

The F15 is clearly an unconstitutional offensive weapon since it could be used to attack other countries, he said.

"The F15 is a very big step outside of the constitution," he added.

Ishibashi also said he considers Gen. Kurisu's remarks to be a sign that civilian control of the Japanese military is weakening because theyrefuted even the government's current interpretation of the constitution.

On the other hand, the Liberal Democratic Party leader most responsible for initiating the current debate strongly denied that his party's goal is an offensive force capable of attacking other nations.

Michita Sakata, chairman of ruling party's security problem research committee, said Japan seeks only a "denial capability," or a force strong enough to protect itself from small-scale invasions. To achieve that, he said in an interview, it needs only more modern defensive equipment, such as the F15s and the new submarine patrol planes, the PC3s, which are also being bought from the United States. Japan also needs improved aircraft-alert systems, he said.

"The basic idea of Japan's defense is passive, to maintain peace," Sakata said. "We cannot be a strong power like we used to be." For power to cope with anything stronger than small-scale invasions, Japan must continue to rely on American forces, he said.

To Sakata, the current debate is valuable not as an argument over types of weapons but as a consensus-building exercise to determine what the Defense Forces should become. They have held an equivocal position in Japan since the war and were held in wide distrust for many years. Sakata produced a public opinion poll showing that 83 percent of Japanese now support the defense system.

The current debate is the result of a confluence of several events that have provoked concern about defense since the early 1970s.

Partly is is caused by the popular perception that the United States is disinterested in Asia and is not the reliable protector it once was. The defeat in Vietnam, the troop withdrawal from Korea, and the Carter administration's emphasis on European defense has spawned doubts that the United States will defend Japan in a crisis.

A Japanese Defense Agency analysis, widely quoted, recently interpreted U.S. defense policy to mean that in a case of a European war part of the 7th Fleet would be dispatched to Europe, leaving North Asia less protected.

Public opinion polls show a declining faith in the American military shield. A recent sampling by the Yomiuri Shimbun found only 19 percent the United States would defend Japan in an emergency compared to 37 percent in 1969.

The new defense concern also stems from a growing fear of the Soviet Union's Pacific fleet, which is based about 700 miles from Tokyo in Vladivostok. Many Japanese interpret that fleet's increasing visibility in nearby waters as a threat to Japan. The Yomiuri's poll showed that 52 percent of Japanese now view Russia as their gravest threat, double the proportion feeling that way in 1969.

Shin Kanemaru, director general of the Defense Agency, has declared that there are so many Soviet ships operating in the Sea of Japan that it could be called "the sea of the Soviet Union." He also said that with only 400 Japanese aircraft facing 2000 Soviet planes in the North Pacific a defense would be like "countering machine guns with bamboo spears."

Fears that Japan's air defense system is inadequate were aroused in 1976 when a defecting Soviet fighter pilot slipped through the radar network and landed on the northern island of Hokkaido.

There are no plans on the books to increase defense spending radically. The Japanese spend nine-tenths of 1 percent of their gross national product on defense - compared to nearly 7 percent in the United States - and the most hawkish enthusiasts talk of spending no more than 1.5 percent.

The United States is encouraging Japan to modernize defensive arms and to pay a larger share of the costs of its forces based here, but is not officially urging Japan to expand its forces.