In 1981, Nissei Realty Inc., the real estate division of the largest Japanese life insurance company, Nippon Life Insurance Co., opened an office in New York and quietly began to investigate the U.S. real estate market.

Soon thereafter, Nissei bought a 50 percent interest in the Pan Ocean Building, a 22-story office and retail structure on Madison Avenue that was owned by Equitable Real Estate Investment Management Inc.

In 1984, six months after Equitable bought the Union Bank Building in Los Angeles for $170 million, Nissei purchased a 50 percent stake in that project.

Late last year, ground was broken on a 50-story building in Dallas that is slated to open in 1987 and is a joint venture by Nissei, Equitable and a Texas developer, Trammell Crow.

These deals are symbols of the growing role of Japanese firms in the U.S. real estate market. Equitable, for example, has teamed up in the last few years with several other Japanese insurance giants, including Meiji Mutual Life Insurance Co. and Asahi Mutual Life Insurance Co., on real estate ventures around the country in which the Japanese interest is almost $500 million.

Equitable estimates that over the next five years, 10 to 15 percent of the $2 billion to $3 billion they invest each year in real estate will come from Japanese sources. This summer Equitable will be opening an office in Tokyo to serve as a liaison with its Japanese partners and to search for new business.

"It's been an evolutionary process," George Puskar, the president of Equitable Real Estate Investment Management, a $23 billion subsidiary of the Equitable Assurance Society, said of his company's ties with Japanese firms. "We had trainees from Japan in the early 1970s nurturing this relationship . . . they're making steps into U.S. real estate like the pension funds did back in the 1970s. They value real estate and they treat it with great respect."

A Nissei executive, who did not want to be named, said that "we wanted to diversify our portfolio and get a higher return on investment."

Many other Japanese firms have the same idea. The Commerce Department estimates that the cumulative investment of Japanese companies in American real estate totaled $1.2 billion at the end of 1985, or almost double the previous year's $630 million..

Asahi Weekly, a Japanese publication, predicts that at the end of 1986 the total investment might reach $4.5 billion. Analysts say that 50 Japanese companies are now investing in U.S. real estate.

"I suspect with the dollar going down, there may be a major increase [in Japanese real estate investment] later this year," said Richard Apcar, director of the Investment Analysis Division at the Commerce Department.

The Japanese investment has been triggered by the accumulation of large amounts of capital by many of the biggest Japanese financial institutions due to recent export surpluses.

The insurance companies leading the investment binge are now being closely followed by Japanese trading companies, banks and construction firms.

For the Japanese, American real estate has a special allure because land in Japan is in very short supply and often the yields from investment are only about 5 percent, as compared with 10 percent in the United States. Further, land and buildings are not sold as often in Japan as in America, where each transaction normally ratchets up the price. Some analysts believe that pressures on Japan from U.S. policy makers to increase direct investment in the United States has been spurring the deals.

Recently, the Japanese ministry of finance added another spur for the Japanese insurance companies when they raised the level of foreign investments permitted to insurance companies from 10 percent to 25 percent of their total assets. Pension funds probably will be permitted a similar increase.

The Japanese Long-Term Credit Bank is setting up a facility called Nippon Landic which will be investing roughly $100 million a year in U.S. real estate, according to an official with the bank in New York.

The Japanese invasion has made that country the most active foreign player in U.S. real estate, replacing British and Canadian investors of a few years ago.

Larry Weber of Chase Investors Management, a division of Chase Manhattan Corp. that has consulted with two of the six insurance companies actively seeking properties in the United States, said this country is especially appealing because it is the "biggest market, the most diverse and the most liquid. Returns here have been higher than other places."

"We're considering increasing our portfolio all over the United States. We're looking at major cities on the East and the West coasts," said Kazunobu Nisawa of Meijiseimei Realty of America Inc., a subsidiary of the Meiji Insurance Co.

Meiji and Equitable own 50 percent of a 10-story office building under construction at 1155 21st St. NW. Meiji's Nisawa said the Japanese firm invested about $30 million in the project, which is being built by the developer Farr Jewett and is scheduled to open in November. When completed, the building will have 300,000 square feet of office and retail space that will rent for $33 to $38 per square foot, according to Farr Jewitt.

Another new venture that is perhaps riskier is now underway in Dallas, where Equitable and Nissei have teamed up with Trammell Crow to construct a 52-story office building due to open in late 1987 and house Texas Commerce Bank.

Other deals are now cooking between Equitable and some of Japan's leading financial institutions. Equitable executives said the giant trusts, Sumitomo and Mitsubishi, have begun to display interest in U.S. real estate, and Equitable is negotiating with a Japanese company about an investment in a regional real estate mall that's under construction.