A reference voltage generating circuit receives a power supply voltage and generates a reference voltage. A reference voltage level guarantee circuit generates a sense signal when the circuit senses that a value of the reference voltage has reached a predetermined value. A power supply voltage sensing circuit has a voltage comparator circuit which compares a voltage obtained by dividing a power supply voltage with the reference voltage and outputs a power ON reset signal. An operation of the voltage comparator circuit is controlled based on a sense signal. When the value of the power supply voltage increases and the value of the reference voltage reaches a predetermined value, the voltage comparator circuit operates, and a power ON reset signal is outputted in response to a result of comparison between a divisional voltage and the reference voltage.