Japan imposed travel restrictions today on Polish diplomats here in retaliation for similar limitations placed on Japanese serving in Poland following Warsaw's declaration of martial law Dec. 13.

Foreign Ministry officials said Polish diplomats in Tokyo would have to receive prior government approval for any trips more than 25 miles from the heart of the city. At the same time, however, the government announced that it would be providing humanitarian aid to the Polish people.

As for instituting any sanctions to register disapproval of the military takeover in Poland--a step strongly advocated by the United States--the government is cautiously watching how far the West Europeans will go.

Officials say a series of sanctions, including some directed at the Soviet Union, were drawn up several weeks ago. Their implementation, an official said, has been blocked by Western Europe's inability or unwillingness to strike a consensus on the matter.

The sanctions issue has confronted officials here with the dilemma of trying to express unity with the tough stand of Japan's key ally, the United States, while warily assessing the moves taken by the Western Europeans.

"It is very important for Japan, as a member of the Western democracies, to keep in step with them," said one official.

The government coupled its announcement today about travel restrictions with a pledge of $500,000 to the International Red Cross for the purchase of food, clothing and medical supplies for the Polish people. The amount is on a par with donations from Canada and Switzerland.

The Japanese justify their caution on sanctions by pointing to what they view as the penalties they have incurred as the result of the sanctions Tokyo imposed on Moscow after the invasion of Afghanistan when Japan moved to restrict trade and diplomatic contacts between the two countries, clamp down on official credits and boycott the 1980 Moscow Olympics.

The scenario of proposed sanctions, which are described here as "parallel with those of the United States," are believed to include, in regard to Poland: a ban on new government-sponsored trade credits, the suspension of efforts to cooperate with Western creditors in rescheduling Poland's official debts this year and the restriction of high-level diplomatic exchanges.

In the case of the Soviet Union, Tokyo is thought to be considering the further tightening of controls on official trade credits already in place since the Soviet invasion of Afghanistan in 1979 and more stringent restrictions on Japan's exports of high-technology goods and the exchange of scientific and technological experts at the official level.

Despite Britain's recent imposition of independent economic sanctions, government and industry sources here indicated that any measures Tokyo puts its stamp on will be closely geared to whatever moves West Germany and France make. "We are waiting for the Germans and the French," said one well-placed source, "but where are they?"

Since Tokyo followed Washington's lead in establishing sanctions against the Soviet Union for sending its troops into Afghanistan, Japan's powerful traders have complained bitterly that they have lost several billion dollars in potential sales of industrial plants to the Soviet Union. Many of these contracts, they charge, have gone to competitors in Western Europe, particularly West Germany and France, where governments pulled back from earlier pledges of support for sanctions.

"In the minds of many Japanese," said one knowledgeable source, "Poland is essentially a European problem, and the feeling is that we shouldn't go farther than the Europeans are willing to go."

Last October, Moscow turned down Japanese bids for $1.7 billion in gas-pipeline compressor stations in favor of West German, French and Italian companies. The Japanese widely interpreted the move as retaliation for Tokyo's alignment with Washington on the Afghanistan issue.

The project is designed to supply Western Europe with 40 billion cubic meters of natural gas from Siberia.

Japan already has a major stake in the project with Komatsu, a major supplier of heavy construction equipment, under contract to sell the Soviets 400 pipe-laying machines, for which the Japanese government has provided $140 million in official suppliers' credits.

Officials acknowledged that any new sanctions Japan puts in force against Moscow are not likely to affect existing contracts with the Soviets. But they suggested that Tokyo would put the clamp on both government-sponsored deals and those worked out on a strictly private commercial basis. "Our policy is not to undermine the measures taken by the U.S. and Europe," an official said.

Meanwhile, the Japanese point out the view that the Reagan administration's unwillingness to impose a new grain embargo against the Soviets has weakened the U.S. position in the eyes of its major allies.