PURPOSE: To eliminate the necessity for providing a vacuum gage, by obtaining a degree of vacuum in a turbo molecular pump from a phase difference between a current and voltage in an induction motor, which drives a rotor, to be displayed.

CONSTITUTION: Voltage is taken out from optional two phases of an induction motor 10 which drives a rotor of a turbo molecular pump, and one of the phase voltage is input to a phase comparator 14. While the other one is input as the current to the phase comparator 14 through a current/voltage converter 16, and a phase difference between the current and the preceding voltage is obtained. An amplifier 18 obtains voltage, corresponding to a degree of vacuum in the turbo molecular pump, from a phase difference signal to be displayed in a display device 20. Accordingly, the necessity for separately providing a vacuum gage is eliminated.