Japanese Foreign Minister Suburo Okita expressed receptiveness yesterday to increasing his country's military spending, as urged by the United States, but made no commitment about when, or whether, such an increase will take place.

Speaking at a news conference at the end of two days of talks, Okita noted "an increasing interest on the part of the Japanese public" in defense.

In private talks with Secretary of Defense Harold Brown, Okita is reported to have made clear that there is divided opinion on the subject within the Japanese government, but that several major departments are backing an increase.

As Okita said publicly, the United States is suggesting a steady and speedy increase in Japanese military expenditures in the light of the Soviet invasion of Afghanistan and the military buildup of U.S. and European powers.

The United States has proposed that the increase be centered in naval and air forces, rather than ground forces, in keeping with Japan's policy against an offensive military posture. Japan is planning to buy almost $14 billion in warplanes from the United States in the next several years.

Japan has held its military spending to less than 1 percent of its gross national product since World War II. One aspect of its defense contribution which is rising -- and probably will continue to rise -- is spending of $730 million annually to support U.S. forces stationed in Japan. s

After talks with secretary of State Cyrus R. Vance and presidential assistant Zbigniew Brezezinski, as well as Brown, Okita met briefly yesterday witn President Carter. In the presence of reporters, Carter said he was pleased that "We are making good progress on trade matters and on our mutual approach to the international challenges we face."

It was the first trip here as foreign minister by Okita, a prominent international economist and non-political figure, who was named to the high post last November. One of his tasks here was to prepare the way for an official visit by Japanese Prime Minister Masayoshi Ohira in early May.

Okita's initial meeting with Vance in Paris last Dec. 10 was marred by public charges from Vance's party that Japanese firms were ignoring Washington's call for an economic squeeze on Iran. Japan Subsequently restrained its trading companies, and the hard feelings on both sides were surmounted.

As a symbol of support for the U.S. response to Afghanistan, Okita announced that Japan will buy an additional 200,000 to 300,000 tons of American grain. This is intended to offset U.S. losses resulting from the partial grain embargo against the Soviet Union.

There was no visible movement in Okita's talks here on the demand by U.S. automobile labor leaders and political figures that Japanese auto companies build plants here. Okita said his country's auto firms may find this "difficult," which is a Japanese way of saying no.