To provide a method and an apparatus for monitoring a power supply of a vehicle callable of notifying an appropriate countermeasure in order to urge a driver or the like by detecting the generation of an abnormal standby current by an on-vehicle electric load device or the like in such a state that an ignition switch or an accessory power supply switch are turned off.

This method for monitoring the power supply of the vehicle is for determining presence or absence of abnormality of the standby current on the basis of a detection current value from a current sensor 11 for detecting an output current of a battery 10 supplying the power to the on-vehicle electric load device. During at least predetermined time after the accessory power supply switch 20 is turned off, a power supply monitoring device 17 supplied with the power from the battery 10 determines the presence or absence of the abnormality of the standby current on the basis of the detection current value of the current sensor 11 when the accessory power supply switch 20 is turned off.