U.S. exports to Japan are expected to grow at least twice as fast as they have in the past few years, a Japanese trade agency predicted.

Shigeru Ishii, public relations director for the Japan External Trade Organization, told the Washington State Export-Import Conference that reduction of Japanese import tariffs and tax breaks for Japanese importers will spur expansion of U.S. exports to Japan.Jetro has not set specific goals for expansion of U.S. exports to Japan, Mr. Ishii said, but it would not be unreasonable for the relatively modest growth rate of the past few years to double.

"I would say we want U.S. exports to Japan to increase as much as possible, but we don't have a dollar figure," he said. "Of course, we'd like to see the growth rate at twice what it is now - at least twice."

Figures compiled by the Commerce Department's International Trade Administration show that the value of U.S. exports to Japan almost doubled between 1984 and 1989, rising from $23.17 billion to $44.58 billion.

That represents an annual growth rate of 14 percent, though real growth each year has varied considerably.

In 1985, for example, U.S. exports to Japan actually fell by 4.2 percent. That turned around in 1986, when exports to Japan grew 20 percent. Expansion in U.S. exports to Japan slowed to 4.5 percent in 1987, then jumped to 35.7 percent in 1988.

Last year's growth in exports to Japan was 18.2 percent, making 1989

gains the closest to the 14 percent average.

To double that average, U.S. exports to Japan would have to rise by $12.48 billion in 1990, boosting this year's exports to Japan to $57.1 billion.

The growth in U.S. shipments will come about as a result not just of government programs but also of a fundamental change taking place in the Japanese commercial system, the Jetro spokesman said.

That system is now oriented toward doing what's best for manufacturers rather than what's best for consumers, he said. Japan has now recognized that it must overhaul regulations and practices to shift the focus to customers.

''In the United States, the consumer is king," he said. "In Japan, the manufacturer is king."

Japan has long recognized the need to switch to a consumer-oriented economic system, he said, but has spent much of the past 30 years trying to determine how it might do so.

''It takes a very long time for us to reach a decision, but once we do, things happen quickly," he said.

Because growth of U.S. exports to Japan has been relatively slow in the past, however, some American exporters at the conference said they have not been holding their breath in anticipation of greater sales to Japan. They noted, for example, that Mr. Ishii had acknowledged that Japan still prefers to buy raw materials such as logs rather than higher-value finished lumber.

The Jetro representative said Japan would rather have the raw logs, but he recognizes that it may have to accept greater amounts of finished lumber.

"We like to import the log itself to the Japanese market, but people on the American side, well, they need a job," he said.

Susan Schwab, director general of the Commerce Department's U.S. and Foreign Commercial Service, said the U.S. government is taking the Japanese government's representations about higher U.S. exports "at face value."

In an interview after a speech to the conference, she said the Commerce Department has drawn up plans to coordinate its promotion of U.S. exports with Japan's promotion of imports from the United States.

She pointed out that opening Japanese markets and then failing to get U.S. companies to sell to those markets would essentially give away the progress made in trade negotiations to companies from Europe, South America and other parts of the world.

Ms. Schwab, who was U.S. trade policy officer in Tokyo in the early 1980s, said the U.S. government doesn't expect massive changes to result from tariff reductions.