An abnormality detection device includes a detection circuit unit, a protection circuit unit, and an abnormality determination unit. The detection circuit unit includes: a voltage detection circuit and a current detection circuit that generate, as analog signals, detection values indicating a current and a voltage at a predetermined position of a vehicle-mounted power supply device. An A/D converter converts the detection value into a digital signal. The protection circuit unit performs a predetermined protection operation on the vehicle-mounted power supply device if a voltage or a current of an input path or an output path of a voltage converting unit has an abnormal value. The abnormality determination unit makes at least one of the detection circuit unit and the protection circuit unit perform a predetermined diagnosis operation, and determines, based on a result of the diagnosis operation, whether the detection circuit unit and the protection circuit unit is abnormal.