Prime Minister Yasuhiro Nakasone made two key concessions on the contentious issue of telecommunications in hastily arranged talks here during the weekend with a special envoy from President Reagan.

One was a private assurance that one or more Japanese employed by a U.S.-affiliated company will be named to an influential council that advises the government on telecommunications policy, according to informed sources here.

The other was a pledge that the number of technical standards for equipment used in Japan's telecommunications network will be decreased in the near future, according to Japanese sources, although not to the extent sought by the United States.

The U.S.-affiliated company is widely believed to be IBM Japan, a wholly owned subsidiary of the American computer conglomerate, which is a major figure in the telecommunications industry here.

The move is meant to help answer U.S. demands that foreign companies be given more say in the debate and drafting of Japanese government rules on telecommunications.

The United States had pushed for foreign representation on the bodies, although recently it has said that the representatives need not be foreign nationals. Japanese law specifies that members must be Japanese citizens.

Nakasone apparently hopes the council and standards moves will signal the United States that he is serious about opening the Japanese market and reducing the U.S. trade deficit with Japan, which last year came to $37 billion.

The United States has long contended that in telecommunications and other fields, rules devised by Japanese ministries without foreign input are a prime means by which local companies are given an unfair advantage.

Getting a voice on the councils is considered an important first step in assuring openness of the decision-making process. If a precedent is established in telecommunications, it might spread elsewhere, it is felt.

However, the key question for many Americans is whether foreign advice will be considered seriously once it is received.

The Japanese government has said that although its laws do not require prior publication of draft rules, as U.S. laws do, it will make strong efforts to see that foreign companies can make their views known.

Nakasone's assurances on appointments to the council involve a delicate area because in theory the councils are neutral, nonpartisan bodies that provide the government with objective advice on issues of national importance.

Private promises that members of a specific company will be appointed could lead to charges from Nakasone's domestic critics that the council's purpose is being corrupted.

There currently are no vacancies on the 20-member body, composed of academics, businessmen and retired officials, meaning Nakasone would have to remove someone to make room for the foreign representative.

The panel in question is the advisory council of the Ministry of Posts and Telecommunications. That ministry has been in charge of drafting sweeping new regulations, put into effect today, that open many areas of the Japanese telecommunications services industry to competition for the first time.

The source said that one Japanese national employed by a foreign-affiliated firm is likely to be appointed to the full council, and another Japanese to one of its technical sub-panels.

Meanwhile, Japanese officials prepared today for further telecommunications talks after a special U.S. emissary sent here to seek new concessions left today for Washington to report to President Reagan.

American specialists are expected to come to Tokyo within days to confer with the Japanese on ways to simplify technical standards that must be met by equipment used in Japan's telecommunications network.

Before leaving Japan aboard a special military jet, the envoy, National Security Council staff member Gaston Sigur, was given a letter from the Japanese government pledging to work toward revising the standards.

Standards questions appear to be the last immediate stumbling block in the telecommunications negotiations, which the two sides have been conducting since January.

Many U.S. officials remain unhappy with the talks' outcome. But there is a feeling among some that, with the exception of standards, the United States has obtained all it can from Japan for now.

Negotiators had hoped to reach agreement on the new regulations by today but were forced to keep talking when agreement on standards remained elusive.

For the present, Japan's own views on the issue have gone into effect as part of the new rules. But the government has agreed to alter them if necessary when the talks with the United States are concluded.

The two sides have formally agreed to wrap up the standards issue within 60 days, but Nakasone yesterday told Sigur that Japan would work to do it sooner.

American negotiators maintain that Japan's insistence that equipment meet approximately 30 different standards in areas ranging from voice level to insulation is unnecessarily restrictive and will tend to limit the market.

Equipment should be judged solely on the basis of whether its use would harm the larger system, they say. Japan, however, maintains that such standards are needed to safeguard overall system quality and to protect consumers.

In a meeting yesterday, Sigur told Nakasone that Japan must give more ground on standards and procedures for foreign participation in rule-making or face protectionist legislation from Congress.

Japanese officials have also agreed to more discussion on rule-making, but they have indicated less willingness to give new concessions in this field and there are no immediate plans to resume those talks.

American and Japanese officials said that Sigur brought no specific new proposals for changes the United States wanted in this field.

Because rule-making affects patterns of decision-making in the entire Japanese government, it is considered a far more difficult subject than the more cut-and-dried question of standards.

Meanwhile, Tokyo companies began registering with the Ministry of Posts and Telecommunications to set up computer network services. They had been highly restricted.

Today, Japan's sole telephone company, Nippon Telegraph and Telephone became a private company after 33 years as a state corporation and lost its monopoly on telecommunications services.

In a related step, the Japan Tobacco and Salt Public Corp., which had a monopoly on distribution of cigarettes, lost it and became a private company today, the first day of the Japanese fiscal year.

These steps were part of a long-term program initiated by Nakasone to sell off state corporations.