Sen. Russell B. Long (D-La.) yesterday questioned the relative competence of negotiators for the U.S. Trade Representative's Office and said they probably aren't as good as their European counterparts.

During a Senate Finance subcommittee hearing on the budget authorization for the trade negotiator's office, Long said the government should find better ways of keeping good people as negotiators and provide more training for them.

Long's remarks came several hours before the Reagan administration confirmed Washington Post reports that an interagency group headed by an official from the Trade Representative's Office will brief the Japanese next week on what the U.S. government plans to do to help the domestic auto industry.

The officials stressed that a briefing, not negotiations, would take place and that President Reagan so far plans no future negotiations on import limits with the Japanese. The president considers the U.S. auto industry's problems a domestic one. No immediate response from the Japanese is expected.

During the hearing, Long said that, in general, "I just happen to believe our people have nothing like the people they're going against . . . They can't hold a candle" to the Europeans. But Long added, "I'm not saying our people are incompetent."

The government needs more experienced negotiators, Long continued. "What we're all doing around here is the blind leading the blind." Government trade "people tell me things, and it's not true at all."

Long addressed his statements and questions to Michael Hathaway, senior assistant general counsel of the U.S. Trade Representative's Office.

"In this area we don't have the competence we ought to have, do you agree?" Long asked. Hathaway said he did not agree. He said, however, that it is difficult to keep people in the trade negotiator jobs as long as their counterparts are kept in Europe.

Hathaway later told a reporter the Trade Representative's Office has 113 employes, including secretaries and chauffeurs. When asked how many of those persons were strictly negotiators, Hathaway said he didn't know because lawyers, economists and others negotiate some agreements. "I've done some negotiating myself," Hathaway told a reporter.

The Trade Representative's Office is asking for $10 million in budget authorization for fiscal 1982. Last year it spent $9.4 million and had the same number of employes it now has.