The Japanese government, in a bid to head off protectionist legislation in Congress, today approved a three-year market-opening plan that includes the reform of 88 regulations said to hamper foreign sales in Japan.

The main points of the "action program" had already been announced. But Japanese officials say new details adopted today will demonstrate further that Japan is serious about addressing trade tensions with the United States.

"The goal of the present decision is to achieve for the Japanese market an openness exceeding that of the international level," Prime Minister Yasuhiro Nakasone said in a statement issued after the plan was adopted.

"Today, I wish to appeal anew to all the Japanese people to willingly accept foreign products," Nakasone said in the statement, which was provided in provisional translation by the Foreign Ministry.

Nakasone said the current international trade situation is an "emergency" and that Japan would work with other countries at the "new round" of multilateral trade negotiations to block the emergence of protectionism.

He said efforts were needed to expand domestic demand in Japan, a step that many trade experts feel would help improve the trade picture. But he did not provide specifics for how it would be accomplished.

The plan was approved at an hour-long meeting at Nakasone's office today of the Japanese cabinet and leaders of the ruling Liberal Democratic Party. Later today, Nakasone is scheduled to go on national television to reiterate his appeal for more imports.

The drafting of today's document, described as an "outline" with many sections still to be filled in, began in April under heavy pressure from U.S. legislators angry over Japan's burgeoning trade surpluses.

The surplus with the United States reached $37 billion in 1984 and is projected to grow as high as $50 billion this year.

This fall, Congress is to consider legislation that would require Japan and three other countries to trim their surpluses with the United States by 5 percent or face punitive tariffs of 25 percent on all their exports starting in 1986.

The Japanese plan approved today leaves unresolved many knotty problems in the two countries' trade relationship, such as agricultural import quotas, activities of foreign lawyers, and membership of a foreign company on the Tokyo Stock Exchange.

On each, the government has pledged to work toward solutions.

Officials said the plan will be closely supervised by a Cabinet-level committee and Nakasone himself.

Many of the steps will require acts of the Diet, or legislature, but that seems secured by the presence at today's meeting of senior leaders of the ruling Liberal Democratic Party as well as Cabinet members.

Japanese officials declined to estimate in dollar terms how today's measures would affect the balance of trade.

Japanese sometimes argue simultaneously that their country is already an essentially open market but that the new steps will give "radical" new access to foreign companies.

Both American and Japanese officials, however, agree that market-opening at best can have a minor effect on overall trade figures.

The dollar's high value, considered to be a problem for the United States to solve, and the competitiveness of Japanese export industries are held to be far more important factors.

Some U.S. officials and business leaders here declined comment pending study of the full document.

American Electronics Association senior representative John Stern, often a severe critic of Japanese trade policy, said: "They have made some positive proposals."

But he said some ministries here continue to oppose liberalization.

A major part of the plan to be adopted today consists of tariff reductions.

They were announced in June and provide for an across-the-board cut of 20 percent or more for about 1,850 product categories.

Japan already had among the lowest average tariff rates among industrialized countries.

By many accounts, the meat of the package is its reform of highly technical standards that products must meet to be sold in Japan.

Foreign companies often complain that Japanese standards are too cumbersome, are administered behind closed doors and can make the cost of doing business here prohibitive.

"The basic principle is to minimize government intervention and to leave the markets in the hands of the consumer," a Foreign Ministry official said. The 88 reforms of regulations fall into this field. According to the Foreign Ministry, the changes will:

*Simplify standards and expand self-certification to more product categories. Producers of these goods would be able to certify independently that their products meet government standards, rather than having the government or private testing agencies do it.

*Expand acceptance of foreign test data for more product types. Currently, Japan insists that most tests be conducted in Japan, even if they have already been done abroad.

*Clear the way for participation for foreign companies on all 22 councils that help devise such standards.

*Simplify import paperwork.

*Simplify auto import procedures. The government will increase the speed at which imported cars are inspected and drop some safety requirements.

Regulations governing approval of cosmetics for sale here will be simplified to the point that the number of licenses issued, currently about 84,000 per year, will decrease by 60 percent over three years, Foreign Ministry sources said.

Licenses for medical equipment will be cut by about 25 percent.

Today's plan also will be expanded to 16 more government agencies to give foreign companies a fairer shot at civilian procurement contracts let by the government. Contracting rules of the General Agreements on Tariffs and Trade will be expanded to 16 government agencies to which it is not required to apply.

In the plan, Japan gives a specific timetable for ending interest-rate regulation for large denomination deposits and for other steps to liberalize their financial markets that had previously been agreed to in principle, Foreign Ministry officials said.

It has also agreed to a new step: a commitment to let financial institutions make certain types of transactions without collateral.

In his television address later today, Nakasone is not expected to give specifics on how he proposes to raise domestic demand in Japan. The ruling Liberal Democratic Party has reached a general consensus on the desirability for more demand but not on how to bring it about.