The European Community Commission demanded Friday that all of Japan's trading partners benefit from Tokyo's pledge to Washington last week to open its market to U.S. companies.

The commission moved quickly to ensure improved market access in Japan for the EC after Tokyo made significant concessions to the United States in the fourth round of the Structural Impediments Initiative talks in Washington."We insist that the results (of the SII) are applied multilaterally," a

commission spokesman said.

Tokyo's concessions to the United States must be granted specifically to all contracting parties to the General Agreement on Tariffs and Trade, he added.

Under last week's bilateral agreement, Japan promised to implement a series of measures ranging from loosening curbs on large retail stores to revising tax policies to free up more land for commercial and residential use.

During a visit to Tokyo last week, Frans Andriessen, EC trade commissioner, secured assurances from Japanese officials that the community would benefit from a more open market, according to the EC spokesman.

Mr. Andriessen had earli- er expressed concern that the current round of bilateral talks between Washington and Tokyo could lead to the United States' securing exclusive concessions.

Despite its hasty bid to share in Washington's windfall, Brussels remains distinctly cool to U.S. attempts to strike bilateral accords with its trading partners. While welcoming Japan's decision to open up its market to imports, the EC stressed that trade problems should be settled within GATT.

The breakthrough in the U.S.-Japan talks is in sharp contrast to Brussels' relative lack ofsuccess in it own attempts to open Japanese markets to EC products. Most recently, the community has sought to link easier access to Japan to opening the EC to imports of Japanese cars after 1992.