Japan, highly successful at exporting cars, computers and calculators, rapidly is increasing its exports of another popular item: cash.

"The Japanese have become a very large supplier of capital abroad," A. Richard Janiak, managing director of Smith Barney Harris Upham International Inc., told a group of business executives Thursday. "Just as they have been dominant exporters of automobiles, the Japanese are emerging as a major source of capital."

Janiak said net investment in non-Japanese securities by Japanese investors has soared from $4 billion in 1980 to approximately $27 billion last year. He said last year's figure was more than double the $13.2 billion recorded in 1983.

The availability of Japanese capital and the interest among the Japanese in buying U.S. corporate securities is good news for corporations here, which eagerly are looking for ways to lower their cost of funds.

Irwin Engelman, chief financial officer of General Foods Corp., visited Japan recently to promote his company's securities. "There is a feeling of excitement and explosive growth," Engelman said.

To emphasize the recent growth and future potential of Japanese investment in securities of U.S. corporations, Janiak began by asking, "Would all of you who have ever eaten sushi, owned a Japanese television or driven in a Japanese car please raise your hand?" The response was close to 100 percent.

"Now," Janiak asked, "would all of you who have been called on by a Japanese bank or securities firm raise your hand?" The response was about 30 percent. In another two years, Janiak predicted, the response to the second question will be close to 100 percent, too.

There are several underlying reasons for the dramatic growth in foreign investments by the Japanese, Janiak said. The Japanese have a very high savings rate. which has led to the accumulation of liquid assets, and those assets can be invested globally. Japan began the process of opening its capital markets to outsiders in 1980 when it relinquished investment and foreign exchange controls, he said.

The Japanese have a lot of cash to invest, Janiak said, citing the dramatic growth in the size of Japanese pension funds and other assets directed by money managers. Growth of these funds has exceeded 20 percent annually and Janiak expects them to increase from $60 billion to about $250 billion in the next eight years.

While U.S. corporations have been able to borrow dollars more cheaply abroad than in the domestic markets, many also are finding they can lower the cost even further by issuing securities denominated in foreign currencies -- such as the yen -- and ending up with dollars by doing a currency swap.

Euroyen bonds, yen-denominated securities issued in Europe and initially sold to investors outside Japan, have been around since 1977. However, until December, only governments could issue them to raise funds.

Following an agreement between the U.S. Treasury Department and Japanese officials, Janiak said, these have become available to corporations as well. In the two months corporations have been able to use them to raise funds, there has been an "explosion" of Euroyen offerings so dramatic that there were more Euroyen bonds issued in December and January than in the previous seven years.

One of the reasons for this is the favorable terms at which corporations could raise capital.