To reduce the applied voltage of an induction motor, by setting both or either one of the input power and input current of the induction motor to the minimum value within such a range that does not exceed a prescribed incremental amount or a set value of the slip of the rotating speed of the motor.

Envelope detecting circuits 14 and 17 respectively find the input current and input power of an induction motor 2 and a slip operator 19 calculates the slip of the motor 2. A conditional voltage generator 21 monitors the slip signal calculated by means of the operator 19, and minimizes both or either one of the input power and input current of the motor 2 within such a range that does not exceed a prescribed incremental, amount or set value of the slip of the rotating speed of the motor 2. Therefore, the applied voltage of the motor 2 can be reduced.