Japan's chief economic planner told reporters here yesterday that his government would like to see a moderate appreciation of the yen against the dollar to assure a continuation of the recent trend toward reduction of the Japanese trade surplus.

Tokusaburo Kosaka, head of the Economic Planning Agency, said "the Japanese economy can withstand a rate of about 190 yen to the dollar." At present, the yen is about 220 to the dollar, a substantial depreciation from the October 1978 peak of about 185 to the dollar.

Some analysts feel that the weakening of the yen against the dollar since that time could again stimulate Japanese exports, reduce imports, and start the Japanese surpluses on a new upward course.

Kosaka said that he expected the "normalization" of the U.S.-Japanese trading relationships "to be sustained in the future." The Ohira government's policy, he stressed, is to encourage the increased imports of manufactured goods, "and I don't expect any major imbalance (in the trade accounts) again."

He indicated that his government would be happy if the yen rate stabilizes somewhere in the 190 to 200 yen range.

Kosaka later clarified his statement, pointing out that under a floating rate system, "the government does not have a target rate for the yen."

Kosaka is in the United States for presummit discussions. He will also go to Paris next week for talks at the Organization of Economic Cooperation and Development that will focus on energy problems.

He acknowledged that the time had come for oil-importing nations to abandon mere rhetoric in dealing with energy problems, and to evolve some sort of effective international cooperation.

When pressed to outline specific measures, he suggested only that consuming nations need to exercise "self-restraint when buying on the spot market" and carry out the pledge made through the International Energy Agency to reduce imports by 5 percent.

Like Secretary of Treasury W. Michael Blumenthal and U.S. State Department officials, Kosaka rejected the notion that a policy of confrontation with Organization of Petroleum Exporting Countries would be useful. Somewhat brusquely, he told a questioner that "any sort of counter measures against OPEC at the moment would be risky." The right policy, he said, is to pursue "rational, reasonable measures" among the consuming nations, poor and rich alike.

He rejected the idea of allocation or self-imposed quota limits on the purchase of spot oil, although he said Japan "is most fearful that the high (spot) prices in the Rotterdam market will spread to other markets."

Kosaka took a low-key position regarding a U.S. decision to pay a $5 subsidy for certain purchases on the European oil market to build up stocks of heating oil. A State Department official Thursday said "it was a mistake" not to have informed other IEA countries in advance.

The U.S. subsidy has been sharply criticized in Europe. But Kosaka said that U.S. officials had satisfactorily explained to him that it was not really a subsidy, but a transfer of payments within the U.S. entitlements program.