MOSCOW, NOV. 13 -- Gennadi Gerasimov, the Soviet Foreign Ministry spokesman whose smooth, straight-talking style made him something of a media star in the West, has left that job to become an ambassador, the Soviet news agency Tass reported today.

The news agency, quoting reliable Foreign Ministry sources, said Gerasimov had been replaced by ministry adviser Vitaly Churkin, a key aide to Foreign Minister Eduard Shevardnadze. Gerasimov, it said, would be named ambassador to an unspecified European country.

Gerasimov's unruffled, straightforward approach and his often caustic or humorous retorts to probing queries was a change from more dour Soviet spokesmen who preceded him and made him a television personality in the United States and Western Europe.

For some years, he accompanied President Mikhail Gorbachev on all foreign trips, but he appeared to lose a measure of prestige earlier this year when the president chose a new spokesman from outside the Foreign Ministry.

Tass said Gerasimov "enjoyed authority and respect as the mouthpiece of official Moscow at a time of radical changes in the country." The news agency said Churkin was "one of the most capable and rapidly progressing diplomats" under Shevardnadze.

Churkin, youthful-looking and affable, was closely involved in forming Soviet policy on the question of German unification, Tass said.