When an image forming apparatus shifts to a power saving mode, a controller unit further reduces power consumption by switching AC current on/off every predetermined amount of time. When AC current input from a commercial AC power supply is turned off, a DC voltage generating circuit normally can no longer supply a DC voltage, rendering the controller unit unable to operate. Accordingly, the controller unit operates using a charge accumulated in a smoothing capacitor while the AC current is not being supplied. An AC cutting circuit is switched on before a both-end voltage at the smoothing capacitor drops below a lower limit of operation voltage of the DC voltage generating circuit, and the smoothing capacitor is recharged.