An inverter control circuit (6) controls the operation of a plurality of switching elements in a three-phase inverter circuit (2) by a PWM signal. A phase voltage output from the three-phase inverter circuit (2) is outputted through a low-pass filter (3). A waveform of the phase voltage output from the low-pass filter (3) assumes the following waveforms through the control of the PWM signal. The waveform becomes zero in a first one-third period of a cycle; forms a sine wave corresponding to a phase from 0 to 2π/3 in a second one-third period; and forms a sine wave corresponding to a phase from π/3 to π in a remaining one-third period of the cycle. Such PWM signal cyclically includes a period where a pulse is not generated. Therefore, the switching action of the switching element is periodically stopped.