When the edge-alignment phase shift is carried out, as shown at the top of FIG. 18, only merely shifting the rising timing of a carrier signal results in that, as indicated by the hatching in PWM2-2, the on period of PWM2 extends into the third control period, so the on period of PWM2 in the second period becomes shorter than a required value. In order to compensate for such a shortened on period within a phase-shift period, an auxiliary carrier having a period corresponding to the amount of phase shift as shown at the bottom of FIG. 18 is inserted in the existing art. By inserting the auxiliary carrier, an on period provided by an auxiliary pulse arises in PWM2 within the second period, with the result that the above-described shortened on period is compensated.

Incidentally, in a period after the edge-alignment phase shift, if the on period of the PWM signal becomes short as in the case of the PWM signals PWM1-3, PWM2-3 shown in FIG. 19, the rising edge of the on period of the PWM signal PWM2 separates (delays) from the falling edge of the on period of the PWM signal PWM1 as in the case of the PWM signals PWM1-3, PWM2-3 shown in FIG. 19.

When the phase shift is executed again as shown at Auxiliary 2 in FIG. 20, an auxiliary pulse based on the auxiliary carrier arises in the PWM signal as surrounded by the broken lines in FIG. 20, so a switching loss due to the auxiliary pulse arises. That is, as a result of execution of the phase shift, a switching loss due to the auxiliary pulse arises.