The Justice Department has launched an investigation into possible predatory pricing of semiconductor products by Hitachi Ltd.

"We welcome the investigation," said Daryl Hatano, spokesman for the U.S. Semiconductor Industry Association. Hiroshi Miyamoto, a Hitachi executive, said the company won't comment until it learns more about the investigation.

U.S. semiconductor firms and government officials have contended that Hitachi of America, a subsidiary of Hitachi Ltd., has sold some microchips below cost to drive U.S. firms out of business.

As evidence, the SIA and government officials point to a memo given to Hitachi microchip distributors this spring that told them to continue to cut prices by 10 percent on one product until they won the bid. Toshi Kitamura, head of Hitachi's international operations, said last week that the memo was the product of the "excessive enthusiasm" of three young American employes and that it did not reflect company policy.

Although the Justice Department's antitrust division does not announce current investigations, Sen. Pete Wilson (D-Calif.) yesterday released a letter he received on Aug. 2 from Acting Assistant Attorney General Charles F. Rule informing him of the investigation. The letter indicated that the investigation had been spurred by a June 6 letter from Wilson and Sens. John C. Danforth (R-Mo.), David L. Boren (D-Okla.), and Frank R. Lautenberg (D-N.J.) that accused Hitachi of violating antitrust law.

The Justice Department would not comment on how long the investigation has been under way and spokesman Mark Sheehan said that it could take anywhere from "a few months to a few years" to complete.

Sheehan noted that only about 10 to 20 percent of Justice Department investigations lead to the filing of an antitrust suit.

The Justice Department investigation is the third federal action concerning Japanese trade in the past two months. On June 14, the SIA filed a petition under section 301 of the Trade Act of 1974, asking that President Reagan consider trade action to stop the Japanese from dumping microchips on the U.S. market and to allow the United States a full share of the Japanese semiconductor market.

Ten days later, Micron Technologies Inc. of Boise, Idaho, filed a trade complaint under Section VII of the amended Tariff Act of 1930, charging the Japanese with damaging the company by dumping microchips on the U.S. market. The International Trade Commission ruled Friday in a preliminary hearing that the company has been damaged by Japanese competition, although the Commerce Department must determine if the Japanese companies used predatory pricing tactics. Final hearings by both agencies must be held before a compensatory tariff could be levied.

U.S. semiconductor manufacturers claim that Japanese companies have gained strength in the U.S. market while shutting the United States out of most of Japan's $8 billion semiconductor market. The SIA says that U.S. companies hold about 10 percent of the Japanese market, but Japanese officials say that U.S. market share there has grown to 19.1 percent. Japan has about 15 percent of the $11.6 billion U.S. semiconductor market, according to the SIA.