Japanese prime minister to urge allies to line up behind Reagan. At summit, Nakasone to talk on Western unity

Japanese Prime Minister Yasuhiro Nakasone will energetically root for Western unity in support of President Reagan at the Western summit that begins here tomorrow. Mr. Nakasone has said repeatedly that Western solidarity behind the United States President is of utmost importance when Mr. Reagan meets Soviet leader Mikhail Gorbachev in Geneva next month. Sources close to the premier say he will speak out forcefully for this viewpoint when six Western heads of government -- from Britain, Canada, West Germany, Italy, Japan, and the US -- meet here.

Nakasone recognizes that putting the spotlight on this issue may help to draw news media attention away from US-Japan trade frictions, a problem he will address in bilateral talks with President Reagan Friday. But he is also a firm believer in Japan's need to move out of single-minded concentration on economic issues and to speak out on some of the world's pressing political problems as well.

``We should move a step forward from an economic role and play a world role as a political power also,'' he told a Japanese group here Monday.

Nakasone himself has reversed the political passivity of his predecessors. He participated actively in efforts to draw up a political declaration at Williamsburg in June 1983, the first Western summit he attended. At that summit, the Western allies, including France, made clear that US intermediate-range nuclear missiles in Europe were a response to Soviet SS-20 nuclear missiles, and that the summiteers would be satisfied with nothing short of a reduction of SS-20s, not merely a transfer of some of thes e missiles from Europe to the Far East.

Williamsburg showed that Moscow could not split the European allies from the US, nor Japan from the Western allies. Western firmness at Williamsburg, Nakasone is known to believe, eventually opened the way for the US-Soviet summit next month.

He believes this, sources close to him say, even though French President Franois Mitterrand has conspicuously absented himself from the New York summit. (Regular summits of the world's top industrialized democracies are held once a year. The next one will meet in Tokyo next May. The New York summit is an ad hoc affair, called by Reagan to take advantage of the fact that most heads of government involved are attending the United Nations 40th anniversary celebrations here this week.)

Nakasone has often said he admires French national pride and wishes Japanese socialists would take a cue from their French counterparts, even though on East-West issues he believes the West has no choice but to stand with Washington.

While emphasizing the need for unity, Nakasone will also argue to his summit colleagues that the Gorbachev proposals for deep mutual arms cuts should not be dismissed out of hand. Whatever Soviet motives may be, this is the first time Moscow has placed such detailed and specific figures on the table.

As for Reagan's Strategic Defense Initiative, Nakasone does not believe Japan has enough solid information to do more than indicate it understandscontinued research by the US.

Nakasone addresses the UN today, and his speech is expected to focus on peace and disarmament, as behooves a country repenting of prewar militarism and ultranationalism and seared by memories of Hiroshima and Nagasaki.

The prime minister is expected to remind his audience that Japan is committed to doubling its economic aid to developing countries over the next seven years, during which time it will disburse $40 billion for this purpose. He will denounce trade protectionism and pledge to make the Japanese market one of the most open in the world. He will call for global efforts to protect the environment.