A control apparatus for producing a pulse width modulating signal to be used for operation of a PWM inverter wherein a modulating wave of a sinusoidal waveform corresponding to the desired AC output voltage of the inverter is sampled at selected periodical time intervals to obtain a level of the modulating wave at each sampling point and the interval is arithmetically determined between each sampling time point and a time point which appears between the sampling point and the next sampling point and at which time point a virtual triangular waveform, which has positive and negative peaks of a predetermined amplitude occurring alternately at the selected periodical time interval, takes the same level as the sampled level of the modulating wave. On the basis of the time interval, the PWM signal is produced.