A DC to DC converter can include a reverse-blocking semiconductor switch that makes a synchronously rectifying MOSFET become parallel-connected with a capacitor that is connected to a power supply of a controller IC for a conventionally used synchronously rectifying circuit. The reverse-blocking semiconductor switch can be driven either by signals for adjusting a voltage of the capacitor within a permitted range of voltage of the power supply of the controller circuit, or by signals that are determined by a signal obtained from voltage across the MOSFET and the signals for adjusting a voltage of the capacitor within a permitted range of voltage of the power supply of the controller circuit.