WASHINGTON — Judge Michael Mukasey was sworn in today as the nation’s 81st attorney general, filling a vacancy left when Alberto Gonzales resigned amid questions about his credibility.

Judge Mukasey was sworn in at a private Justice Department ceremony about 16 hours after he narrowly won Senate confirmation. The oath was administered by Assistant Attorney General Lee Lofthus, who oversees the department’s management and budget operations.

A Justice Department spokesman, Brian Roehrkasse, said Judge Mukasey was joined by family members at the brief, closed-door ceremony. The ceremony took only about two to three minutes, and a small coterie of Justice staffers who attended it clapped afterwards, Mr. Roehrkasse said.

A public ceremony is being planned for next week at the White House.

Judge Mukasey, the third attorney general of the Bush administration, inherits a Justice Department widely seen as struggling to restore its credibility with more than a dozen vacant leadership jobs and little time to make many changes before another president takes office.

After taking the oath, Judge Mukasey headed immediately into meetings with senior Justice Department officials. Among them was his top national security adviser for a briefing about the Foreign Intelligence Surveillance Act.

His first full day on the job will be Tuesday after spending the holiday weekend at his home in New York, Mr. Roehrkasse said.

Judge Mukasey served nearly two decades as a federal district judge in Manhattan, including six as its chief jurist. He oversaw many of the nation’s highest profile terror cases in the aftermath of the September 11, 2001 attacks.