A phase shift amount for the overlap phase shift is set in correspondence with fluctuation predictions in on period. Each fluctuation prediction can be carried out by obtaining rates in the above-described feedback control. FIG. 9 shows an example of the overlap phase shift. Initially, an on period (Duty2₀×Rate2^t) of PWM2_t after an overlap period t (in FIG. 9, t=4) from a current period (PWM2₀) is obtained. The PWM signal PWM2 is shifted by a half of the on period (Duty2₀×Rate2^t)/2 and a half of the on period (Duty1)/2 of the PWM signal PWM1. Specifically, the phase shift amount θsft is expressed by the following mathematical expression (2).

$\begin{array}{c}(\frac{\mathrm{Duty}??1}{2}+\frac{\mathrm{Duty}??2_0\times \mathrm{Rate}?}{}\end{array}$