A differential amplifier is used to determine the level of an output voltage of a charge pump circuit, and is operated in the voltage follower mode with a reference voltage being an input thereto for charging a capacitance element with a resultant output voltage of the amplifier circuit. Subsequently, a voltage to be compared that corresponds to the output voltage of the charge pump circuit is compared with the voltage stored in the capacitance element to generate an output signal by the differential amplifier. According to this output signal of the differential amplifier, the charge pump operation of the charge pump circuit is selectively activated. A power supply circuit is provided which stably generates an internal voltage at a desired voltage level.