American officials have insisted that a Japanese businessmen's trade mission be prepared to place substantial orders for goods in the United States during its forthcoming American, Japan's top foreign trade official said yesterday.

"We are told by the American side quite a number of times that they expect real business to be concluded by the mission." said Nobuhiko Ushiba, the foreign trade minister.

He also said the businessmen's mission expects to make "a dramatic statement" presumably about buying American goods - near the end of the trip.

Ushiba, speaking to foreign reporters at a lunch, declined to confirm a report that the U.S. special trade representative, Robert Strauss, had demanded that the trading mission purchase a specific dollar amount of goods from American manufacturers.

Japan commited itself last month to expanding its imports as part of a broader effort to reduce its heavy trade surplus, a major irritant in U.S.-Japanese relations. Ushiba's comments yesterday, along with the reports of Strauss' reported demands, indicate that U.S. officials are making blunt demands on the Japanese to prove their good intentions.

About 90 leading Japanese importers will leave here Thursday for a tour of U.S. industries that has been promoted by the Japanese government as a step toward improving trade relations. It is designed to convince American businessmen that the Japanese market can be open to them.

Until recently, the mission had been portrayed as exploratory and not designed to make specific purchases.

Reports circulated here that Strauss had telephoned Ushiba and suggested that the mission be cancelled if the businessmen were not prepared to buy during the trip. A Japanese source confirmed that Strauss wanted such assurances but had only suggested to Ushiba that the mission be postponed, not cancelled. With so many high ranking executives involved, it was decided that the mission could not be postponed, he said.

Ushiba would not say exactly what Strauss told him, but he emphasized that his governconsiders the mission's "long-term benefits" to be more important than any immediate purchases it might make.

Themission, headed by Mitsui and Co. president Yoshizo Ikeda, will visit San Francisco, Chicago, New York, Washington and other cities. It hopes to pay a call on President Carter.

Ushiba, who is the principal Japanese negotiator on international trade, also said that his country is beginning to change internal practices that foreign businessmen have claimed prevent them from selling goods here.

These businessmen complain that Japanese customs practices and a cumbersome distribution system discourage them from exporting to Japan even when tariffs and quotas are not impediments.

Middle-level business managers are willing to make changes to accomodate foreigners. Ushiba said, but he acknowledged that government bureaucrats such as customs officials are slow to change their old-fashioned ways.

"There are people at the windows [customs officials] who have been in the past accustomed to a more rigid kind of attitude toward imports," Ushiba said. "This is also changing but we must have a little more patience."

Ushiba was asked if it would be necessary to wait until such officials retire and are replaced by new personnel. "Yes," he said with a laugh, "to a certain extent that is true."

The trade minister said he is familiar with the American arguments that large Japanese exports create unemployment in U.S. industries, but he added, "I personally feel that this argument is tenable more in theory than in fact."

"We must not let emotional arguments get the better of us," he said, "Either we will prosper by combined efforts . . . or we will stagnated in isolation."

Ushiba held out little hope for the European Economic Community's trade problems. Representatives from the EEC are in Tokyo this week attempting to negotiate trade concessions to alleviate the large imbalance of trade with Japan. The Common Market has threatened Japan with higher tariffs if concessions are not forthcoming.