The package of sanctions adopted by Japan today is less severe than the one agreed on by European countries and avoids making a direct threat of a total trade embargo against Iran.

But the government hopes to use it as the basis for a strong plea to President Carter to avoid a show of force to free the 50 American hostages in Iran. Prime Minister Masayoshi Ohira will carry that message to Carter during a brief state visit in Washington next week, officials said today.

They contended that by risking the long-term loss of Iranian oil and other economic penalties, Japan and European countries can now fairly insist that Carter exercise caution.

Support for any sanctions has been weak here, in part because Japan is more vulnerable than other countries to Iranian retaliation. Yet the fear that the Carter administration is preparing military action is strong and officials do not hide the feeling that it would have disastrous consequences.Ohira has publicly justified sanctions as a step needed to avoid the American use of military force.

"We feel that by sacrificing the oil, we have earned the right to urge self-restraint," one official said today.

The Ohira government decided at a morning Cabinet meeting on the basic outline of sanctions and its spokesman said that in principle Japan plans to cooperate with nations in the European Economic Community which acted earlier in the week.

The EEC foreign ministers' resolution threatened a total trade embargo against Iran if there is no "decisive progress" toward freeing the hostages by the middle of May.

The Japanese government's agreement carried no such ultimatum on a total trade boycott. Chief Cabinet Secretary Masayoshi Ito told reporters the government will wait and see what the European nations decide and then discuss what concrete steps it will take.

Officials said the absence of an ultimatum did not mean Japan would refuse to go along with a total boycott if the European countries come to that point. They said, however, that Japan hopes for a less severe outcome.

The government did propose a halt to new export and service contracts which would at least keep Japan's trade with Iran from increasing and ultimately would diminish it considerably. That trade has been growing swiftly since last fall and exports to Iran reached $173 million in February. s

The Ministry of International Trade and Industry this afternoon began advising companies not to sign new contracts. Such "administrative guidance" from the ministry sometimes is followed by industry and sometimes not and officials emphasized it is not binding.

Japan agreed to reduce the 16-member embassy staff in Tehran by "several" persons. Officials said they would be transferred one at a time and the date of the first one's departure was not announced.

However, the government decided not to force a cut in Iran's representation in Tokyo. Ito said the four-member Iranian Embassy staff here is too small to justify a reduction.

A major question remaining is what Japan will do about the large petro-chemical plant being built in southern Iran by Mitsui and Co. and four other Japanese companies. Officials gave conflicting signals today but it appeared they hope to exempt it from any trade sanctions to appease the Iranians.

The petrochemical plant is 85 per cent completed but work has been stalled since March and the Japanese companies have been warned by Iran to get on with construction or face the possiibility of having it expropriated with a huge loss of money.

The petrochemical plant is 85 per-cent completed, but work has been ect despite the American pressure to keep the work there only at a level of limited maintenance as part of the economic pressure campaign against Iran.

One government official said today the government's policy has changed in the light of recent pressure for sanctions and that it is now "undecided" on whether to encourage Mitsui and others to resume construction next month as planned.

Iran has consistently used the Japanese investment in the plant as a point on which to discourage Japan from siding with the United States on sanctions, threatening to turn the project over to East European technicians. s

News reports from Tehran tonight quoted Iranian Finance Minister Reza Salimi as warning that the plant is now in jeopardy. He reportedly said that Iran would never ressume joint projects with countries that went along with the United States even if the hostage issue is solved.