Even before arriving here Monday on his first official visit, Japan's Prime Minister Yasuhiro Nakasone has made a highly favorable impression on Reagan administration officials.
It has been less than two months since Mr. Nakasone took power in Japan, but the administration has clearly decided that his vigorous leadership offers a major opportunity to strengthen the US-Japan defense relationship and to resolve economic disputes between the two nations.
Washington has never seen a Japanese prime minister quite like this one. Whereas most of Japan's post-war prime ministers have been cautious men, Nakasone has shown a willingness to take risks and brave criticism which has impressed US officials. They cite several of his initial actions as evidence of this. Within a matter of only a few weeks, Nakasone has:
* Initiated new tariff cuts and market-opening measures.
* Managed to secure a larger increase in Japan's defense budget than had seemed probable before he came to office.
* Broken a 16-year-old ban on the export of Japanese military technology in order to allow the export of such technology to the United States.
* Reached agreement with South Korea on long-term Japanese economic aid through a precedent-breaking trip to that country.
Emphasizing the need for good relations between Japan and South Korea, America's northeast Asian allies, a senior administration official described Nakasone's opening to South Korea as his most dramatic move so far.
''What all this adds up to is a very good and strong start,'' said the official at a briefing for reporters at the White House.
He added, however, that the administration still has ''some very hard problems'' to discuss with Nakasone during the Japanese leader's visit here this week.
The official, who spoke under an agreement that he not be quoted by name, said that in the economic field, the US feels that there is still ''a very long way to go'' before US products enjoy the same freedom of access to Japanese markets as Japanese products enjoy here. The official said that protectionist sentiment was growing in the US and that prompt action by the Japanese was required.
In the field of defense, the official said that the administration does not feel that Japan's defense spending increases are adequate to provide for the defense missions which Japan has pledged to carry out.
As a result, the official said that the administration anticipates ''tough, frank discussions'' when Nakasone meets with officials here. He is scheduled to meet with President Reagan Jan. 18.
One aim of the Reagan-Nakasone meeting is to build personal rapport between the two leaders, and they are expected to get along well. They are both open, amiable men who like to talk about major issues without getting bogged down in detail.
For the Japanese, there is a ritualistic aspect to the meeting.All Japanese prime ministers are expected to come here and show the Japanese people that the alliance is in place and that they have the ear of the American president.
Nathanial B. Thayer, director of Asian Studies at the Johns Hopkins University's School for Advanced International Studies, says he thinks that over the long run, the new Japanese prime minister has a chance to consolidate the strategic alliance between Japan and the United States just as the late Chancellor Konrad Adenauer consolidated the NATO alliance. Dr. Thayer, a Japan expert, has been assisting Nakasone in the writing of a book.
Nakasone has placed great stress in his first days in office on the importance of Japan's relationship with the US.