Japan's new finance minister, Tatsuo Murayama, said here over the weekend that his country is sticking to its target of reducing the 1978 current account surplus to $6 billion, despite an embarassing bulge in that surplus since the $6 billion target was set in January.

He presented a case for Japanese economic achievement and a genuine effort toward international collaboration that will be reiterated this week by Prime Minister Takeo Fukuda when he meets with President Carter in Washington.

Murayama, making his first appearance on the international scene at the International Monetary Fund's interim committee meeting, was responding to unexpectedly sharp critism not only from the IMF but also from the Organization for Economic Cooperation and Development.

Both IMF managing director H.J. Witteveen and OECD director general Emil Van Lennep singled out Japan and West Germany by name at a Saturday meeting of the Group of Ten industrial nations for maintaining large surpluses and failing to meet domestic economic growth targets.

Witteveen said that the recent appreciation of the yen and the German mark were welcome developments from an international standpoint, but that they must be supported by stimulation of domestic growth in both countries.

Murayama reviewed the measures being taken by the Japanese government "to meet our international responsibilities," including extensive deficit financing "to which we have dared resort despite criticism within Japan."

He repeated pledges made to U.S. trade representative Robert S. Strauss that Japan would open its markets to imports, and would participate fully in multilateral negotiations in Geneva. But he warned that the "massive surplus" in the Japanese current account would be reduced only slowly.

"I am convinced," Murayama said, "that for fiscal 1978 (which began April 1) various effects will begin to be seen, and the effects of yen appreciation will be evident in a lower volume of exports and increased imports into Japan."

He said that the trend would begin to be visible especially after September, when special conditions - including a push by Japanese exporters to beat the effects of appreciation of the yen - will largely have been concluded.

At the time of the Strauss negotiations in January with Japanese ambassador Nobuhiko Ushiba, the Japanese estimated that the current account surplus for fiscal 1977 would run $10 billion, and that it could be sliced to $6 billion this year. But the 1977 surplus turned out to be $14 billion.

Murayama told reporters here that the $6 billion, nevertheless, is still the target for this year. "We would like to aim at the figure, and come as close as possible."

Murayama would not speculate on the future level of the Japanese yen, which appreciated about 20 percent against the dollar in 1977. "But I think there is a general feeling among the Japanese people that it has come to a fairly stable level at the moment."

On the Japanese growth rate, the government is sticking to its announced 7 percent target, although the estimate for actual growth last year has now been shaved back to a little more than 5 percent, Murayama said. Most private Japanese economic research agencies doubt that Japan economic growth this year can be much more than the 5 percent level of last year. But most observers at these sessions accept at face value Murayama's assurance that Japan is approaching the problem "with its best effort and sincerity."