Japan will purchase a modified version of a U.S. jet as its next-generation fighter-bomber rather than build a competing aircraft, Japan's chief defense official announced here yesterday.

Yuko Kurihara, director-general of Japan's Self-Defense Agency, announced the decision to select either the McDonnell Douglas F15 or the General Dynamics F16, ending months of tension and suspense between Tokyo and Washington about the issue.

Kurihara, who spoke to reporters after informing Secretary of Defense Caspar W. Weinberger of the decision, said his government will attempt to make final decisions about the new plane before Prime Minister Yasuhiro Nakasone leaves office late this month.

The decision was immediately hailed by U.S. political figures who had applied heavy pressure for Japan to choose a U.S. plane to assist U.S. industry and help offset the massive U.S. trade deficit with Japan.

Sen. John C. Danforth (R-Mo.), whose state is headquarters for McDonnell Douglas and General Dynamics, was the first to break the news in the United States with a press release calling Japan's move "a major positive development" that could presage "a turning point in U.S.-Japan relations."

Danforth took credit for "pressing for a 'made in U.S.A.' decision" and said he had intervened with legislation as well as conversations and correspondence with leading U.S. and Japanese officials.

Senate Majority Leader Robert C. Byrd (D-W.Va.), who cosponsored with Danforth a provision of the omnibus trade bill calling for a tough U.S. line on the fighter-plane issue with Japan, called the decision "the right choice in terms of advancing our common security interests and in terms of easing the frictions in our relationship."

Japan, whose military activities are limited to defense under the terms of its 1947 "no war" constitution, plans to use the new plane primarily for air defense and interdiction of hostile ships, according to U.S. military officials.

If final selection and manufacture proceed promptly, the new plane will enter the Japanese air fleet in about 10 years, the officials said.

The U.S. aircraft are expected to be "lightly modified" with special Japanese radars and other high-technology devices, according to the U.S. officials.

An unspecified portion of the manufacture will be performed in Japan under license from the winning U.S. firm. Japan expects to spend $6 billion or more on the new plane over the next 20 years.

Officials and engineers inside the Japanese defense industry and the Self-Defense Agency had been pushing for a basically Japanese plane, saying that the technology exists in Japan and should be encouraged. On the other side were the Foreign Ministry and others concerned with U.S. relations and, in the end, Nakasone.

U.S. officials said Kurihara had also told Weinberger yesterday of a decision to undertake an ambitious U.S.-Japan effort to improve the ability to detect Soviet submarines. The effort is likely to cost Japan $200 million to $300 million, including a new antisubmarine warfare research center and construction of antisubmarine surface ships resembling huge catamarans, the officials said.

Antisubmarine technology has been a subject of intense discussion since a subsidiary of Japan's Toshiba Corp. was found to have exported metal milling equipment to the Soviet Union in violation of export controls. The Pentagon has charged that the Soviets are able to use the equipment to fashion quieter submarine propellers, making detection more difficult.