Japan's auto shipments to the United States will slow down sharply during the balance of the year in an apparent attempt to ease the tension between the two countries over the auto import issue, a Japanese trade official said in Tokyo yesterday.

According to Japanese press accounts, Trade Minister Rokusuke Tanaka said Japan's automakers probably will export fewer cars to the United States in the balance of this year than they did in the fourth quarter of 1979. By contrast, the United States imported 1.3 million Japanese cars in the first half of 1980, up 40 percent over the year before.

One source said that Japan's Ministry of International Trade and Industry is expecting a 2 percent reduction in car exports to the United States in the final three months of this year compared with the same period in 1979 and has reason to believe the Japanese automakers will comply.

Whether Tanaka's statement reflects such a commitment or is merely a prediction of slower sales by Japanese automakers was not clear, however. Carter administration officials would not comment on Tanaka's statement until after an official translation has been received and studied.

Japan's leading automakers, Toyota Motor Co., Nissan Motor Co. and Honda Motor Co. have resisted firmly previous pressures by the MITI to accept export restraints.

Instead, officials of Toyota and Nissan, which manufactures Datsuns, have tried to sidestep the issue by predicting a slowdown in their U.S. sales this fall. The two companies say they expect sales for the second half of 1980 to run at the same pace as in last year's second half because of the weakness in the U.S. economy and an aggressive sales campaign this fall by the Big Three American manufacturers. Of Japan's Big Three, only Honda is predicting bigger sales this fall than last.

A voluntary commitment to restrain shipments would be a long step past such pessimistic predictions, however, because it could limit Japanese exports even if overall car sales rebound this fall and American consumers still aren't switching wholesale to U.S.-built models.

Press reports from Tokyo said Japanese auto industry leaders were "aware" of the angry reaction in the United States over their exports and were closely watching an investigation of the issue by the U.S. International Trade Commission.

The commission will hold a hearing next month to determine whether the plant closings and unemployment that have rocked the U.S. industry were due primarily to competition from Japanese imports or equally to other factors such as a shift in consumer preferences to smaller cars or the sharp rise in gasoline prices. A decision is not expected until after the presidential election in November.

If the commission concludes that Japanese-built autos are the chief reason for the plight of the U.S. automakers, President Carter could take a range of actions to restrain imports from Japan.