Japan's trade minister apparently reversed himself yesterday and said in Tokyo no decision has been made about continuing to limit exports of cars to the United States past next March.

The question of extending export restraints into a fourth year "is a completely clean slate," Sosuke Uno, head of the Ministry of International Trade and Industry, told the Foreign Correspondents' Club of Japan.

Uno's latest statement appears in direct contradiction to remarks he made June 30 when he told leaders of Japan's auto industry that Tokyo would not extend the quota on auto exports to the United States past March. "The voluntary restraint will not be extended," he said flatly at that time, drawing strong cries for trade protection from Capitol Hill.

There were suggestions at that time that Uno, who had just become trade minister, had spoken more strongly than his government would have liked in appearing to rule out any continuation of the export quotas. There was also speculation that his comment then was the opening gun in bargaining designed to increase the Japanese export quota above its present limit of 1.68 million cars a year.

With American car sales climbing, Japanese auto makers are chaffing under restrictions on their sales in the United States. Many of them have complained that the quotas are preventing them from retaining their share of the lucrative U.S. market.

That limit was set in 1981 for two years, but was extended by Japan into a third year last March. At that time, U.S. Trade Representative William E. Brock strongly recommended it be continued for a fourth year to give American car makers more time to recover from the recession and to remove the question of a future extension from the American presidential campaign.