PURPOSE: To enable a magnetic noise of an AC electric motor which is connected to a three-phase PWM inverter to be reduced without increasing a switching frequency.

CONSTITUTION: A modulation rate corresponding to a three-phase voltage command is input and then an offset value which minimizes an evaluation function expressing a constituent of a high frequency of voltage between output lines of a three-phase PWM inverter 1 is obtained. A value which is obtained by subtracting the offset value from the modulation rate is compared with a triangular wave at a comparator 5 as a new modulation rate and then a switching signal controlling three switches of the PWM inverter 1 is output. A frequency of the triangular wave is controlled according to a value of the evaluation function.