To an output of an NMOS having one end connected to a power source, a capacitor and a PMOS are connected. A capacitor is connected to the output of the PMOS. The NMOS and the PMOS are turned on alternately. A pulse is applied to other end of the capacitor which is connected to the output of the NMOS, to shift the output of the NMOS for boosting. Then, a back gate of the NMOS is connected, via a PMOS in an on state, to the power source. With this structure, the PMOS provides a resistor component when the output terminal short-circuits.