To monitor the CPU computation time in a power converter online, and if the computation time is determined as being abnormal, output pulses is interrupted and further, inform a higher-level processor is informed of that.

A method for monitoring the computation time in the power converter involves a PWM data computing means 2, which calculates the output voltage command according to a frequency command received from a higher- level processor 1 and calculates the degree of modulation of signal waves corresponding to the fundamental wave component of output voltage from the frequency command and the output voltage command and a phase angle which advances by each cycle of carrier waves that is used as the basis of switching, and a PWM pulse generating means 3 which calculates the pulse width of PWM pulses, based on the degree of modulation; and the phase angle which are the output of the computing means and generates PWM pulses. The CPU computation time for the PWM data computing means is monitored by the PWM pulse generating means 3.