Masayoshi Ohira, the prime minister of Japan, has been in government service for four decades and has held almost every important public office. Yet to all but his closest friends, he remains an enigma.

Ohira's private life is a guarded one, his public views indistinct. One official selected toe explain Ohira to foreigners recently began his effort by saying, "Even in Japan, it is said that he is hard to understand."

Ohira, 69, will make a state vist to Washington this week, his first foreign journey since becoming prime minister last December in an unanticipated victory of political muscle over then-incumbent Takeo Fukuda.

Americans will see a stocky, portly man with a bulldog face, a stiff public manner, and a pattern of speech so agonizingly slow that each word seems painfully drawn from his throat. In private, friends say, he is articulate and wryly witty but he dislikes public flashiness and modulates his own mannerisms so as to seem determinedly dull.

Personal modesty is almost a religion with Ohira. "To be perfectly honest, I cannot say say that any achievement of mine is worth boasting about," he wrote in 1970 essay after serving in the centers of power for nearly 20 years. "I have somehow muddled my way through each and every duty allotted to me in public and private life and managed to get through them all without committing any serious mistakes."

The attitude extends to his view of government, which he firmly believes is limited in what it can do for the public good. No politican should hope to do all of what he plans, Ohira says frequently. If he accomplishes only 60 percent of it, that is good enough.

He is found of quoting a leader in Mongolian history whose motto was: "Better to eliminate one wrong than to initiate one right." To a fellow Cabinet minister, Ohira once passed on this advice: "The nation needs a minister who will make every effort to eliminate one wrong far more than it needs one who works diligently to initiate one hundred rights."

The prime minister has charted no new domestic courses since taking office but has promised to alter the pace of Japanese life.

"We have to live in an age of deceleration," he said in a recent interview, "and we can no longer aspire merely to raise our standard of living. We must look for a more leisurely pace of life and a sense of tranquility in our life style."

The rare glimpses offered of Ohira's private life hint at a sensitive intellectual who loves books. He has recently read works by American intellectuals Daniel Bell, Ezra F. Vogel, and Peter F. Drucker. Before becoming prime minister, he made it a point to vist a bookstore twice weekly, buying several volumes, usually of history or social essays.

In an uncharacteristic display of passion, he once described a sense of exhiliration at picking up a new book. "The crisp smell, the special feel of these ne books as they are bought in fresh off the press each week,make me supremely happy," he wrote. "It is moments like these that make me glad to be alive."

Like most Japanese intellectuals, he is profoundly concerned about lack of spiritual identity in his country, which he sees as wandering ceaselessly between the cultures of the West and China. Japan has borrowed liberally from both. "But the conflicting currents of foreign learning have not quenched our spiritual thirst, and our anxiety and uncertainty continue to mount as we remain unable to find safe harbor for our thought and way of life," he has written.

His habits are Japanese to the core. One of his hobbies is calligraphy, the delicate gentleman's art of painting Chinese characters, and a publicity phto shows him at home in a kimono bent over his brush and white paper. Although a Christian by choice as a young man, Ohira has adopted the usual prime ministerial practice of worshipping at Shinto shrines on special occasions.

Ohira was born in 1910, the son of a poor farmer on Shikoku, one of the four main Japanese islands. Like most successful modern Japanese politicians, he rose through the bureaucartic ranks to become an aide and friend to an important public official whose sponsorship raised him to high positions.

A minor economic official during World War II, Ohira was never drafted into the imperial armed services, a fact he now regards thankfully. His Tokyo home was destroyed by American fire bombing and he has described his feeling at the end of the war as one of vast relief that it was over.

Ohira's first prominence came as an aide to a postwar prime minster Hayato Ikeda, whose campaign he had helped to manage. He has held the position of foreign minister twice, and has also been finance minister, minister of international trade and industry, and secretary general of the Liberal Democratic Party.

Ohira has met briefly once before with President Carter - in 1975 when Carter, then governor of Georgia, attended a metting in Tokyo of the Trilateral Commission. At the time, Carter was a dark-horse candidate seeking the Democratic nomination. According to one government official's account, Carter ended his meeting with Ohira with the words: "Let's meet next time at the White House."