President Reagan begins a three-day visit to Japan tomorrow but the Japanese increasingly are unclear about the rationale for the trip. Originally intended to cover five nations, the presidential tour has shrunk to Japan and South Korea through the elimination of stops in Indonesia, the Philippines, and Thailand.
This has caused great disappointment in the Japanese Foreign Ministry, which was looking for a strong presidential affirmation of an American commitment to what an official called ''the bright future of the Asia-Pacific region.''
This theme no longer seems so relevant, but no adequate substitute has been found.
In the meantime, Prime Minister Yasuhiro Nakasone has decided to dissolve the Diet (parliament) and to call a general election around Dec. 18. The decision, announced Nov. 2, seems intended to break the parliamentary deadlock caused by the refusal of former Prime Minister Kakuei Tanaka to resign his Diet seat after his conviction last month in the Lockheed bribe scandal.
Public sentiment against Tanaka, Nakasone's mentor and colleague, will have lessened by mid-December, some observers say. Even so, within the ruling Liberal Democratic Party there are predictions it will lose at least 20 seats, though it will still have a majority.
Another consideration is that a December election will make it easier for the government to take what are likely to be unpopular decisions early next year on the economy and new budget.
Nakasone's election decision also appears linked to the Reagan visit, which should strengthen the prime minister's profile as a statesman. Foreign Ministry sources speak vaguely about using the Reagan visit for ''image building'' purposes, as well as educating the Japanese public about the need for this country to play a larger global role.
According to an official: ''We want to impress the Japanese people with the good personality of President Reagan, showing that his image here as a trigger-happy hawk is too simplistic.'' The invasion of Grenada has complicated that scenario.
The presidential visit from Nov. 9 to 12, therefore, is described as a ''media event.'' But Japanese newspapers have pointed out strongly that Mr. Reagan's schedule provides virtually no chance for him to be seen by the Japanese public.
Exposure will be limited to an address to the Diet (parliament) and an all-channel television interview that, officials say, will avoid substantive issues.
Government officials are hoping the talks between Reagan and Nakasone will promote the friendly ''Ron and Yasu'' relationship that has sprung up through two meetings earlier this year. This is seen as a major political asset in preventing the various problems between the two countries, such as trade frictions and security issues, from getting out of hand.
Two rounds of formal talks, each of an hour's duration, are scheduled. But the two leaders will also have a chance to talk over lunch on two occasions, the second a ''families only'' get-together at Mr. Nakasone's mountain retreat outside Tokyo in the town of Hinode.
Yet nothing substantial is expected to emerge from these various encounters. No joint communique or press conference is planned. It is hoped, a Foreign Ministry source said, that the talks will not bog down on specific issues but will cover mainly international affairs.
One reason for this hope is that Mr. Nakasone will not have very much to offer his guest. On various bilateral trade problems the only breakthrough occurred this week when US Trade Representative William Brock managed to win a Japanese commitment to a fourth (and almost certainly final) year of self-restraint on auto exports to the American market. In return, the ceiling was raised from 1,680,000 units to 1,850,000.
But Mr. Brock was unable to make any progress in other key areas, particularly the expansion of Japanese imports of US farm products like beef and oranges.
Japanese sources say Mr. Nakasone will be unable to offer President Reagan any encouragement, as the agricultural issue is closely linked to domestic political developments.
One question is whether the President will be able to accept the continued weakness of the yen, which has defied all the Tokyo government's strengthening measures and is a prime cause of an American trade imbalance with Japan this year that may exceed $22 billion.
Japanese financial and monetary authorities are hoping the package of economic measures announced by the government Oct. 21 designed to bolster the yen and liberalize local capital markets will sufficiently convince Washington of Japanese goodwill.
Yet even within the government there appear to be strong doubts whether the measures will actually work.
Government sources say one difficulty is that US demands to boost the yen's value while at the same time stimulating imports are contradictory. Says Finance Minister Noboru Takeshita, ''(It's) like telling us to turn on both the air conditioning and the heating at the same time.''
One possible bright spot is the bilateral energy trade. Some sectors here believe the presidential visit offers an opportunity to reach broad agreement on the Japanese import of Alaskan oil and natural gas, for example. It may also be possible to give some sort of commitment to expand imports of US West Coast coal in the long term.