When the sWitch S1 is in an ON state (the state Where the MOS transistor is ON), its drain-source is substantially short circuit. To turn on the sWitch S1, a voltage higher than a predetermined threshold voltage must be applied to betWeen a gate and the source. That is, a voltage higher than the input voltage Vin must be generated to turn on the sWitch S1. In this circuit, a voltage higher than the input voltage Vin is generated by a boost capacitor CB, Which is charged When the voltage of a node A suf?ciently drops. Namely, the boost capacitor CB is charged When the sWitch S1 is OFF (the MOS transistor is turned off) and at the same time, the sWitch S2 is ON.