For the second time this week, the Commerce Department found yesterday that Japanese companies are using unfair trade tactics to gain dominance in the U.S. semiconductor market.

The preliminary finding came in the first semi-conductor complaint ever brought as part of President Reagan's efforts to uproot unfair trade practices, and is likely to intensify pressure on the government of Prime Minister Yasuhiro Nakasone to end Japan's below-cost pricing of semiconductors and to assure American companies equal access in his country.

It also holds special significance because Commerce Secretary Malcolm Baldrige said any penalty duties assessed in this complaint will apply to future generation of semiconductors.

With Japanese companies facing four major complaints accusing them of unfair trade practices in semiconductors, high technology has emerged as the key trade irritant between the United States and its major Asian ally.

Commerce Undersecretary Bruce Smart said U.S. and Japanese negotiators will hold another round of talks here later this month in an effort to settle the range of semiconductor trade issues. He said the Japanese are coming here "with a positive attitude" after a set of "constructive" talks in Tokyo last week.

Smart acknowledged that Nakasone's meeting with President Reagan here next month and the economic summit in Tokyo in May impose added pressure on the negotiators to reach a settlement. But he added that the Reagan administration is prepared to handle each complaint separately, including the imposition of retaliatory duties in a case brought by the domestic industry charging predatory pricing here and restricted sales in Japan.

In addition to the government talks, U.S. and Japanese industry leaders will meet in Los Angeles this weekend to see if they can work out a deal that will guarantee American companies a fair share of sales in Japan and end what U.S. producers call Japan's "predatory pricing" in this country.

Although the meeting is billed as a private one, called by Sony Corp. Chairman Akio Morita and Motorola Corp. Chief Executive Officer Robert Galvin, the Japanese government will have high-level representation there. The Reagan administration declined to send a representative, Smart said, because of possible antitrust implications.

Yesterday's decision concerns the largest-selling semiconductors, 256K dynamic random action memories ORAMs), which Commerce investigators found were being sold at prices far below their fair market value, he legal defination of dumping. It it is upheld in a final ruling due in late May, yesterday's findings could more than double the price of Japanese-made chips in the United States.

Semiconductors, also known as memory chips, are key elements in computer and telecommunications systems. The United States started as the world leader, but Japan began to take over that market in the late 1970s.

"At least six U.S. plants and production lines have been shut down in the past year within the semiconductor industry, and overall employment in the industry has dropped 20 percent," said Commerce Secretary Malcolm Baldrige in announcing yesterday's finding.

He said industry losses could of 256K DRAMs.

In the findings released yesterday, Commerce investigators found the biggest offenders were NEC Corp. and Mitsubishi Electronic Corp.

As a result of the preliminary findings, these companies will have to depost with the U.S. Customs Service a cash bond on all imports equal to dumping margins, determinded by a complicated formula.