In an isolated DC-DC converter, an on-period control circuit generates an off-timing signal when the output voltage of an isolated DC-DC converter exceeds a reference voltage. A signal reception/power switch driving circuit causes a first switching device to be turned on based on a pulse signal for switching that is output from a PWM control circuit, and causes the first switching device to be turned off based on an off-timing signal transmitted by an off-timing signal transmission unit. A bootstrap circuit boosts a control voltage of the first switching device with the pulse signal for switching that is output from the PWM control circuit.