PURPOSE: To provide very precise PWM control without using a high- performance processor or special timer by using the interruption timer function or software timer function of a microcomputer according which is larger, the specified time not shorter than the interruption system conversion time or pulse width.

CONSTITUTION: A one-chip microcomputer 8 makes the difference between the pulse width which was so calculated as to be used for PWM control of an inverter 3 and the time which corresponds to the specified time not shorter than the interruption system conversion time which is the time till the interruption handling is started after an interrupt signal is given. When the microcomputer 8 discriminates that the pulse width is larger, it sets a difference between the calculated pulse width and the value which corresponds to the interruption system conversion time in an interruption timer 8g. When the microcomputer 8 discriminates that the pulse width is not larger, it sets the calculated pulse width in a software timer 8h. Consequently, a very precise pulse width modulation control is provided without a special timer function.