To suppress the ripple of power voltage even if there is a large load variation at drop of capacity of a smoothing capacitor.

A capacitor deterioration detector 306 detects the capacity drop of the smoothing capacitor 22, and an FF control system 309 feeds forward the control state of a load 40 to a constant voltage switching circuit 20 in the case that the capacity drop is detected, so as to make a switching element 202 perform the switching control, whereby the variation of output voltage Vo is suppressed, without causing a delay in the correction to the load variation by the switching control of the switching element 202 tinged with feed forward of the control state of the load 40, even if there is a large load variation at capacity drop of the smoothing capacitor 22.

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