White House national security adviser Richard V. Allen acknowledged yesterday that a Japanese journalist gave him $1,000 intended as thank-you money for an interview with Nancy Reagan, and that the cash was put in an office safe and forgotten for eight months.

Allen's story, supported by White House officials who maintained that no wrongdoing had occurred, features a Justice Department investigation that could lead to the appointment of a special prosecutor, the president's wife, a Japanese women's magazine, a lot of bit players the White House refuses to name, and an alleged Japanese custom of paying important figures for granting press interviews.

The day began with White House officials forced to respond to news reports from Japan that Tokyo police had cooperated with a U.S. investigation of an unnamed high White House official. A major Japanese newspaper, quoting diplomatic sources, said in a front-page article that former Japanese government officials, business and academic associates of the unidentified White House official were questioned in what was described as a bribery investigation.

Presidential counsel Fred Fielding drafted a statement saying that an FBI investigation had found no law or regulation broken. White House deputy press secretary Larry Speakes and communications director David Gergen told reporters that Allen had been cleared of any suspicion.

Gergen told reporters the matter was straightforward. By the end of the day it seemed more rather than less confused, however.

The Justice Department quickly contradicted the White House statements, explaining that its investigation of how Allen got the cash and what happened to the money over eight months is still under way.

Gergen then apologized. He explained that Fielding had written his statement without talking to the FBI or the Justice Department.

The incident began Jan. 21, the Reagans' first day in the White House, when a Japanese magazine called Shufu no tomo (Housewife's Companion) was granted an interview with Mrs. Reagan. Gergen said he could not explain why the First Lady had been ready to give Housewife's Companion the coup of an interview so soon after the inauguration. Speakes said it lasted only about five minutes.

Allen was involved in the interview, he conceded, but he left vague what his role had been. Allen, who as a business consultant before entering the Reagan White House had some Japanese clients, denied that he arranged the interview and Gergen said Allen does not know who set up the brief session.

Allen was present to be photographed with the interviewer, Fuyuko Kamisaka, and a small group of other Japanese.

According to Fielding's account, the Japanese group informed Allen "of their intention to tender an honorarium" for the interview.

"Knowing this to be customary in Japan and not wishing to embarrass the Japanese journalists or the First Lady, Mr. Allen received the honorarium and gave it to his secretary for safekeeping until he could ascertain the proper procedure for turning it over to the government," the Fielding statement said.

Allen was working in an Executive Office Building office at the time and when he moved to his permanent office in the basement of the White House West Wing he forgot the envelope, he said.

"I suppose you might think it's hard to do," Allen told reporters of forgetting the cash, "but we did it in this case." He said two secretaries whom he would not name also knew of the cash but forgot about it.

Allen said he was well aware of the regulations prohibiting White House officials from accepting gifts of value. "I certainly am, that's why I put it away for safekeeping," Allen told reporters in a brief encounter.

He and Gergen also insisted on a semantic distinction.

"I didn't accept it. I received it," Allen said of the envelope containing the $1,000.

"He did not accept the money," Gergen said. "Legally there's a difference."

Reagan was told about the incident and the investigation it has triggered for the first time yesterday by presidential counselor Edwin Meese III, Gergen said. Mrs. Reagan also learned of the incident for the first time yesterday, Fielding said.

"As far as I know there is no evidence of any wrongdoing," the president said to reporters as he left the White House for a trip to Texas. Asked whether Allen would stay on the job, Reagan replied, "On the basis of what I know, yes."

Fielding and Gergen said the money was discovered in mid-September when the safe was opened. It had remained in the Executive Office Building office which was being used by three people working for Allen on the national security staff, Gergen said.

Gergen declined to say who opened the safe and said he did not know how the money was traced to Allen or whether anything was written on the envelope.

Gergen also could not answer reporters' questions about who notified the FBI of the money's discovery or exactly when the FBI investigation began. Fielding said the FBI conducted interviews in Washington and Japan after it was brought into the case.

The FBI has completed one report, but the continuing Justice Department investigation was automatically triggered under the Ethics in Government Act of 1978, which requires a preliminary probe of any allegation of illegal activity by a federal government official. If after a preliminary investigation that cannot exceed 90 days the attorney general finds further investigation necessary, he must ask the U.S. Court of Appeals to appoint a special prosecutor.

Justice Department spokesman Thomas DeCair said that Attorney General William French Smith was notified of the investigation in mid- to late September and that his preliminary investigation would be concluded well before the end of next month. He had no comment on reports from Japan that the FBI had sought Tokyo law enforcement authorities' cooperation in interviewing businessmen and former government officials who were clients of Allen's consulting firm.

Gergen belittled the importance of the episode when he first answered questions yesterday.

"I guess we don't find this whole story quite as exciting as you do," Gergen said as reporters pressed for details he did not know.

"Dick is very familiar with the customs of Japan," Gergen said in explaining why Allen was not surprised that the money was offered. Fumio Matsuo, Washington bureau chief of Japan's Kyodo News Service, said yesterday: "This is absolutely not a custom."

The interview at the heart of the episode lasted only about five minutes, according to Speakes.

Matsuo called it "extraordinary...strange...unfamiliar" behavior for a writer to take $1,000 into the White House for an interview with the First Lady Mrs. Reagan.

Yoshihisa Komori, on leave at the Carnegie Foundation from his job as a Mainichi Shimbun reporter, said: "We don't give money to public officials for interviews." He said former government officials occasionally are given fees by journalists who interview them after they have left office.

Allen was asked if he had ever received such a payment before. "I don't believe I ever did," he replied. "I can't recall ever having done so."