A semiconductor integrated circuit device includes a power supply circuit that generates one or more internal supply voltages from an external supply voltage, and one or more functional circuits that operate on the one or more internal supply voltages. A step-down converter in the power supply circuit generates one or more stepped-down voltages from the external supply voltage. A control circuit in the power supply circuit compares the external supply voltage with a reference voltage and selects the internal supply voltages from among the external supply voltage and the stepped-down voltages according to the result of the comparison. The semiconductor integrated circuit device can accordingly operate on different external power supplies, and can continue to operate on battery power even if the battery voltage drops.