President Carter and Japanese Minister Masayoshi Ohira agreed today to make the subject of Indochinese refugees a major issue when they meet here later this week with other leaders from Western Europe and Canada.

At Carter's and Ohira's instructions, Secretary of State Cyrus R. Vance and Japanese Foreign Minister Sunao Sonoda met later today and were scheduled to meet again Tuesday to discuss in more detail possible collective action to relieve the plight of more than 300,000 refugees who are scattered across Southeast Asia.

American officials insisted that there are no plans to present a joint American-Japanese proposal on the refugee question at the economic summit conference of the Industrial democracies that begins here Thursday.

Yet the president cleary was seeking Japanese support for additional commitments by the seven summit nations to provide assistance to the refugees, and administration officials traveling with him seemed pleased with the initial Japanese response.

Largely at Carter's initiative, the refugee question has emerged in the last week as second in importance only to energy among the issues that are to be discussed at the economic summit. In addition to the United States and Japan, the other summit countries are Britain, France, West Germany, Italy and Canada.

While the summit is unlikely to produce specific pledges by any of the countries, the president is expected to seek a public commitment from the others to do significantly more in terms of the numbers of refugees they accept or the financial aid they provide for the refugees or both.

He also is expected to ask the other leaders privately to exert their influence on the government of Vietnam to halt its wholesale expulsion of people from that country, which is the main source of the refugee crisis.

The subject of refugees came up this morning - on a day otherwise largely devoted to ceremonial activities - as Carter and Ohira met at the prime minister's residence and later had lunch together.

There had been speculation in Tokyo that Carter might make specific new demands that the Japanese either agree to admit more Indochinese refugees as permanent residents or contribute more funds to the international resettlement effort.

However, informed Japanese sources said the presidential made no specific requests on either point. They also emphasized that Carter made no criticism of Japan's performance in admitting refugees, and extremely sensitive subject for Ohira.

The Japanese officials said Carter told Ohira that the refugees issue is not just a problem for Japan and the United States, but should involve their five allies who are coming to Tokyo later in the week.

The Japanese initially had not wanted the emotional refugee issue to play an important part in the summit discussions. In briefing reporters today they emphasized that it still was not listed as a formal agenda item for the talks.

Japan has been the target of persistent foreign criticism for its failure to admit more refugees. So far it has legally accepted only six Vietnamese under rules that require family relations in Japan or a willing sponsor to guarantee them jobs. Japan has more recently agreed to accept up to 500 refugees permanently, but officials contend the country lacks room to settle large numbers.

The Carter administration is seeking congressional approval of funds to raise the number of refugees admitted to 7,000 a month.

Japan is the second largest contributor of funds to the office of the United Nations high commissioner for refugees and has promised to make more funds available. Ohira was quoted today as saying that Japan will provide at least one-fourth of the funds needed to construct a large center on some Southeast Asian island that would serve as a temporary haven for refugee "boat people."

According to U.S. officials, Carter and Ohira also agreed today to raise the questions of world hunger and technological development in Third World countries during the economic summit.

Little else was made public about the Carter-Ohira talks during the first day of a three-day state visit to Japan that is viewed here as largely ceremonial prelude to the economic summit conference.

Offering a toast at the lunch, Ohira gave for hime today, Carter indirectly referred to the differences between Japanese and American society that have much to do with the two countries' different attitudes toward the refugee problem.

"Yours is one of the most homogenous people, so closely bound together that you can almost communicate with one another without even speaking," Carter said to Ohira.

"Ours is a nation of immigrants, of refugees, extremely different one from another, coming from all nations on earth, with different languages, a different heritage, different background, different interests, but still bound together in one nation, deriving strenght because we have a common goal and a common purpose."

Tonight, Carter attended a state dinner given by Emperor Hirohito. In a toast Carter declared that the prosperous nations that will be represented at the summit "have the resources, the skill and the dedication" to overcome such problems as world hunger and energy shortages.

During the day, Carter visited Tokyo's Meiji shrine, a Shinto memorial to the emperor who reigned in Japan from 1852 to 1912, and the nearby iris garden.

[On Tuesday, Carter resumed talks with Ohira, this time at the seaside town of Oiso, 40 miles from Tokyo, news services reported.] CAPTION: Picture, President Carter and Prime Minister Ohira share a laugh at the start of their discussion yesterday. UPI