PURPOSE: To displace the natural frequency of load driven and the vibrational spectra of noises, and to prevent the generation of noises by changing the relational expression of the frequency of a command wave and the frequency of a carrier wave on specified output frequency.

CONSTITUTION: Voltage corresponding to set frequency output from a controller 7 is frequency-converted by a V/F converter 6, and counted by a counter 4 and input to address ports A0∼A8 for a storage cell 3. The voltage is A/D converted simultaneously by an A/D converter 5, and input to address ports A9∼A12 for the storage cell 3. Outputs D0∼D6 corresponding to input values to the address ports are emitted to the storage cell 3, and a switching element controls the number of revolution of a motor 1 by PWM in response to the emission of the outputs D0∼D6. Carrier frequency is controlled by inputs to the address ports A9∼A12, and the ON/OFF pattern of PWM is determined by force to the address ports A0∼A8.