New Japanese proposals governing purchases of telecommunications equipment appear unlikely to allow American companies access to the Japanese market equal to the access Japanese firms enjoy here, according to a key administration negotiator just back from Tokyo.

"We've gotten some idea of what they are going to do, and we don't like it," said Clyde Prestowitz, special counselor on Japan to Commerce Secretary Malcolm Baldrige.

The issue of equal access for American companies in Japan's telecommunications market, the second-largest in the world, is especially sensitive since President Reagan made it a central issue during his Jan. 2 meeting with Prime Minister Yasuhiro Nakasone. In return, the Japanese prime minister promised "to create an environment where foreign firms can compete against Japanese firms on the same footing -- on equal footing."

But the first round of followup talks to the Reagan-Nakasone meeting indicated that the Japanese are dragging their heels on ensuring Americans equal access to Japanese markets, Prestowitz and other government and industry sources reported.

Administration officials said the United States could shave at least $10 billion off its $36.8 billion trade deficit with Japan, the largest with any country, if American companies had the same access to Japanese markets that Japanese companies have in this country.

In telecommunications, for example, American manufacturers last year sold $110 million worth of equipment in Japan, while Japanese companies exported $1.5 billion worth to the United States.

Prestowitz said current indications are that Japan will place a heavy layer of bureaucratic controls over purchases of value added networks (VANs), a system that allows computers to talk to each other over telephone lines and a piece of the market where U.S. companies are considered the best in the world. Doing so could lead to discrimination against American manufacturers.

"That's 180 degrees from the way we treat the Japanese in this market," Prestowitz said.

This view that American exporters face unequal treatment around the world, but especially in Japan, has spawned cries for a new toughness in American trade policy from business interests and key congressmen. Sen. John C. Danforth (R-Mo.), chairman of the Senate Finance Committee's trade panel, is circulating a resolution saying any relaxation of restrictions on Japanese car imports should be linked to "substantially increased" sales of U.S. products in Japan.

Four years of auto restraints end March 31, and a Cabinet-level committee is considering whether to recommend that President Reagan ask that they be continued.

Talks concerning another area of trade friction with Japan -- restraint on its steel exports -- start today with Japanese negotiators taking a hard line, insisting that the United States has increased its demands since a general agreement was reached in early December.

But the main arena for negotiations centers on telecommunications because of a new Japanese law turning the government-owned monoply, Nippon Telegraph and Telephone Co., over to private ownership and allowing for competition. The law takes effect April 1. The underlying regulations currently are being drawn up in Tokyo, and U.S. negotiators are trying to make sure the regulations give American companies an equal shot at sales.

While the Japanese have refused to show the Americans actual drafts, saying they are still being formulated, extensive talks showed the new regulations set up a revamped system for inspection and certification of products for sale in Japan. This system allows the government to monitor and control a company's sales through its distribution of seals needed before a product can go to market.

"It's the metal baseball bat all over again," said Prestowitz, referring to Japanese controls that kept U.S.-made softball bats out of its market for years.