PURPOSE: To resume the leak test effectively by lowering the background level at a mass spectrometric section quickly even when a large quantity of He is mixed into an oil in an oil-sealed rotary vacuum pump due to gross leak.

CONSTITUTION: When a measurement at a mass spectrometric section ANAL exceeds a threshold level, a microcomputer unit 4 judges that gross leak has occurred, and controls a solenoid valve GBV fixed to the gas ballast port 21 of an RP2 to open for a predetermined time. Consequently, atmospheric air is introduced through the gas ballast port 21 into the RP2 and He bubbles in the oil are stirred to be discharged quickly through the RP2. Since no He is discharged again from the RP2 at the time of leak test after occurrence of gross leak, the problem of increasing background level at the mass spectrometric section ANAL is solved.