To prevent displeasure due to a sudden change of electromagnetic sound being generated from an inverter, even if a switching frequency changes suddenly at modulation switching among 2-phase and 3-phase, when a 3-phase voltage type PWM inverter of variable speed for driving a motor is switched among 2-phase modulation and 3-phase modulation.

A switching state signal generating unit is prepared in an output side of a 2-phase modulation operating part. The sudden change of electromagnetic sound accompanying the switching is alleviated, by generating an intermediate state signal during a predetermined period from the switching state signal generating unit to output the summing port, when a switching command signal from 3-phase modulation state to 2-phase modulation state is generated from outside.