An abnormality detection circuit monitors a power supply voltage and, when the power supply voltage drops, outputs an abnormality detection signal of a predetermined level. The source of a detection transistor as a P-channel MOSFET is connected to a power supply line to which a power supply voltage to be monitored is applied. A detection resistor as an impedance element is provided between the drain of the detection transistor and a ground terminal. A capacitor is provided between the gate of the detection transistor and the ground terminal. A charging path is provided between the gate of the detection transistor and the power supply line. The abnormality detection circuit outputs a drain voltage of the detection transistor as an abnormality detection signal.