TOKYO (AP) _ Prime Minister Sousuke Uno, chosen by the governing party as a ''Mr. Clean'' to rescue it from scandal, was kept busy Wednesday denying he might resign because of allegations about his sexual conduct.

In exchanges with reporters throughout the day, Uno repeatedly denied reports that, in meetings Tuesday night, leaders of his Liberal Democratic Party had to persuade him not to resign over of allegations he kept paid mistresses.

Stock prices plunged despite the denials.

According to the reports, Uno told party leaders the accusations insulted his dignity and made it difficult for him to meet other world leaders.

''Political Situation Appears Increasingly Fluid,'' said a headline in the national newspaper Asahi, and the financial newspaper Nihon Keizai declared: ''Omen of Confusion in Politics.''

Uno said Wednesday ''I'm not such an irresponsible person'' as to resign before making political reforms he promised on taking office June 2. He told legislators and businessmen at a trade meeting he would attend the summit of seven industrialized nations in Paris next month.

He was foreign minister when he replaced Noboru Takeshita, who resigned to take responsibility for the scandal involving Recruit Co., an information and publishing conglomerate accused of trying to buy influence with bribes and cut-price stock.

Reports about Uno's alleged ''woman problem'' have incresed in the past week.

Mitsuko Nakanishi, a former geisha, said on television Sunday the prime minister, who is married, paid her to be his mistress in 1985. The same allegation had appeared in a national magazine, without naming Ms. Nakanishi.

Two weekly magazines said later that Uno kept two other mistresses.

One reported that a former geisha known professionally as Hatsuko said she was paid 100,000 yen ($700) a month for 10 years until 1985. The other quoted his former secretary as saying that, from 1962 to 1972, Uno paid an unknown sum to a Ginza bar hostess identified only as Ms. Y, who became pregnant and had an abortion.

Foreign exchange dealers attributed a sharp decline in the value of the yen Wednesday to fears of political instability. The dollar closed at 143.55 yen, up 2.32 yen from Tuesday's close.

Daiwa Bank's Kenji Ishizuka said most of a 0.67 percent drop in the main Tokyo Stock Exchange index was due to ''politicians' inability to resolve their problems.''

After Takeshita decided to resign because of the Recruit scandal, which led to the resignations of three Cabinet ministers, the party spent a month searching for a scandal-free successor. Uno was chosen after Masayoshi Ito, a noted reformer, refused the job.

In Parliament earlier this week, Uno would not answer questions about the alleged affair with Ms. Nakanishi, calling it a private matter. On Wednesday, he repeated his previous comment that ''I've never done anything contrary to morality.''

Women's groups have demanded that he explain his actions, threatening to campaign against his party otherwise. The Liberal Democratic Party has governed Japan since it was formed in 1955.

The Liberal Democrats were shaken Sunday by a defeat in an upper house by- election at Niigata in northeast Japan, which is traditionally conservative but elected a woman running for the Socialist Party.

Elections for the Tokyo Municipal Assembly are scheduled for the weekend and half the upper house of Parliament is to be chosen next month.

On Wednesday, an Asahi editorial on Uno's problems declared: ''It is bizarre indeed that this man is being forced, for personal reasons, to virtually hide from the voting public right before important elections. He is totally lacking in class.''