Japan and the United States initiated a new trade agreement today that is intended to give American telecommunications industries better opportunities in the Japanese market.

Although details are incomplete, the agreement accepts the principle of "mutual reciprocity," which means each nation's businesses should have equal access to the other's markets.

It is intended to settle, at least temporarily, the long controversy over Japan's willingness to accept foreign bids for its government purchases. The dispute was shelved last month to prevent it from spoiling Prime Minister Masayoshi Ohira's state visit to Washington.

Robert Strauss, U.S. special trade representative, and his Japanses counterpart, Nobuhiko Jshiba, initialed the agreement today and at a news conference hailed it as a step toward resolving the two countries' longstanding trade problems.

Strauss asserted that the agreement would guarantee "complete reciprocity" by the beginning of 1981, when a new international code governing bids on government purhcases is to go into effect.

American officials portrayed inclusion of the phrase "mutual reciprocity" as a significant concession by the Japanese and one certain to satisfy American congressmen who believe Japan has enjoyed unfair trade advantages in the past.

While the agreement does not spell out how much of Japan's telecommunications market is to be opened, it establishes a system for working out details that would assure equal access to markets in both countries. Those negotiations are to begin in July and mist be completed by December 1980.

American negotiators originally insisted that Japanese government purchases totaling $7.5 billion be opened to foreign bids. Japan has agreed to open up $6.9 billion but so far has resisted opening Nippon Telephone and Telegraph's more sophisticated computer and switching equipment of foreign bids. Nippon Telephone is a semi-autonomous government utility.

One American source estimated today that if the details are worked out equitably an additional $300 million worth of Nippon Telephone's purchases would be opened up for bidding by companies in the United States and elsewhere. Nippon Telephone buys almost all of its equipment, from telephone poles to computers, from domestic suppliers. It has claimed that foreign-made equipment such as computers and switching gear would not be compatible with its system.

Strauss said today that the original Japanese offer had been unclear about what Japan was opening to foreign bids. It might have amounted to "buying a pig in a poke," he said.

The future negotiations will enable American industry to determine just what is open for bids in Japan. Strauss indicated that if one segment of the Japanese market is determined to be closed a similar portion of the Amercian market would be closed to Japanese bidders.

"We will attempt to achieve, and will achieve, full reciprocity by the time the [international procurement] code goes into effect," he said.

The agreement and the elaborate staging of its announcement here are partially aimed at domestic politics. U.S. negotiators wanted some evidence of reciprocity to show to Congress as a way of winning approval of the Tokyo round of international tariff agreements and of heading off threats to place new restrictions on Japanese imports to the United States.

Ohira's government, in turn, wanted the divisive procurement issue out of the way to smooth the path for President Carter's state visit and the seven-nation economic summit here late this month. CAPTION: Picture, Prime Minister Ohira and U.S. Trade envoy Strauss confer in Tokyo. UPI