A low-voltage detection circuit detects a low voltage using a voltage follower type operational amplifier and an A/D converter instead of a conventional low-voltage detection circuit. That is, a reference voltage is applied from a reference voltage generating circuit to the A/D converter through the voltage follower type operational amplifier. The voltage follower type operational amplifier is used to reduce output impedance. The power supply voltage can be detected by a converted value (a digital value) from the A/D converter since the reference voltage is independent of the power supply voltage and the converted value varies depending on the power supply voltage. The converted value (the digital value) from the A/D converter is set in a register and statuses of a microcomputer are set as in the conventional art, using the converted value as a flag.