The American subsidiary of Mitsui and Co., the giant Japanese trading company, pleaded guilty yesterday to all 21 counts of an indictment charging it with conspiring to evade U.S. duties on illegally priced steel sold here.

Mitsui agreed to pay $210,000 in criminal fines, the maximum allowable, and $11 million in civil penalties, a figure negotiated by Justice Department lawyers and Mitsui attorneys to cover the wrong done to the U.S. government, as part of a plea arrangement, the Justice Department said.

In return, the government agreed to stop its investigation of the parent, Mitsui & Co. of Japan, and to refrain from bringing more charges to the grand jury against Mitsui in America, the Justice Department said.

Also charged in the indictment were three Japanese, Katsumi Arai, an official in the steel and wire products division of Mitsui; Tsuneo Namiki, a general manager in the steel department of Mitsui in the United States, and Takeo Teraoka, deputy general manager and a salesman in the San Francisco office of Mitsui. The employes are expected to surrender for arraignment in the next two weeks. However, a Justice Department spokesman said he didn't know whether they would also plead guilty.

The indictment of Mitsui follows indictments of two major Japanese electronics companies this month for violations of U.S. trade laws. Hitachi Ltd., Mitsubishi Electric Co. and some of their employes are accused of attempting to steal computer secrets from International Business Machines Corp.

The Mitsui plea ended an 18-month investigation of charges that Mitsui and the employes conspired to hide sales of steel at prices below fair market value to avoid the Antidumping Act of 1921 and the trigger-price mechanism, a system intended to monitor steel sales and trigger a government investigation when steel import prices drop below specified levels.

If steel is sold below the trigger price and is found to have injured domestic steel makers, the government can levy stiff duties against the offending importer.

The indictment said the American subsidiary, Mitsui (U.S.A.), used $1.3 million in kickbacks, credits and secret rebates to disguise the true price of the steel. The company provided false information to U.S. Customs agents through the rebates, submitting false damage claims by American customers to Mitsui (U.S.A.) and secret commission payments to the foreign parent company of an American customer, the indictment said.