Former Senator Mike Mansfield arrived to take up his post as ambassador today, with the Japanese hailing his appointment as a symbol of the high importance President Carter places on U.S. relations with this country.

Mansifed, 74, said that he was carrying Carter's message that the closest possible ties with Japan are "the cornerstone of American foreign policy."

The former Senate majority leader said that he shares the President's view that the best possible relations between Japan and the United States are "an essential part of any effort to solve the problems facing the world community."

In the early 1970s, relations between the two countries were strained by the so-called Nixon shocks, especially the U.S. move to open relations with China without consulting Japan and the U.S. restriction on the export of soybeans, a staple of the Japanes diet.

A frequent visitor to Japan, Mansfield is recognized here as an authority on Asian affairs. He taught Asian history at Montana State University and was a member of the Senate Foreign Relations Committee.

The Japanese hope that his understanding of Japan's situation, his access to Carter and his influence with Congress will enable Mansfield to ease trade and energy disputes between the two countries.

Earlier this month, Mansfied predicted that he would have a difficult task as ambassador here because of bilateral economic problems.

One major disagreement Mansfield is certain to be involved in immediately is the future of a Japanese nuclear reprocessing plant scheduled to open next week. Negotiations for a compromise between Japanese nuclear-energy plans and Carter's policy on limiting plutonium technology have not met with success.

As an early spokesman for the withdrawal of U.S. ground forces from South Korea, Mansfield will have to calm Japanese fears of possible instability on the Korean Peninsula.

The major area of friction is likely to be commerce - Japan's $5 billion trade surplus with the United States last year and the advance of protectionism in America.