PURPOSE: To make it possible to select an arbitrary V/F characteristic by installing a means to detect the variation of the current of the DC intermediate circuit of an inverter, a judgement means, and a PWM signal control means to decrease the variation by a certain frequency when the variation is beyond the level range and to increase the variation by a certain frequency when the variation is less than the level range.

CONSTITUTION: When the output current of an inverter oscillates, a current variation detection circuit 11 detects the oscillating current component thereof as a current variation. A judgement circuit 12, which is a window-type comparator, transmits signal (b) when input signal (a) is larger than VH or signal (c) when 2<-VL. A PWM signal control circuit 13 normally finds output voltage V by calculation according to the setting of frequency and transmits a signal generated by controlling voltage V by PWM control. When the current oscillates, the amplitude of signal (a) is beyond the level range of the judgement circuit and signal (b) or (c) is entered to the PWM signal control circuit 13. When signal (b) is entered, the output frequency thereof is lowered by correction frequency f predetermined according to a motor characteristic and the signal is transmitted.