A power circuit abnormality detection method for being applicable to a power circuit and for detecting whether or not an abnormality in a power source relay exists, the power circuit comprising the power source relay which opens and closes a connection between a direct current power source which drives an electric motor and a motor drive circuit which includes a plurality of switching elements and which supplies a drive current to the electric motor; a smoothing capacitor which is connected between a power source wire between the power source relay and the motor drive circuit and a ground wire to absorb ripples in current; a pre-charge circuit which opens and closes a connection between the direct current power source and the smoothing capacitor by bypassing the power source relay to pre-charge the smoothing capacitor; and a discharge circuit which is connected between the power source wire and the ground wire in parallel to the smoothing capacitor and which discharges electric charges stored in the smoothing capacitor via a discharge resistance when a discharge switch is closed, the power circuit abnormality detection method characterized by the steps of: switching on the pre-charge circuit to charge the smoothing capacitor to a voltage (Va) of the direct current power source; switching on the discharge circuit after charging the smoothing capacitor to the voltage (Va) of the direct current power source; switching on the power source relay; switching off the pre-charge circuit after switching on the power source relay, and starting a monitoring timer to count the discharging time of the smoothing capacitor; detecting, after elapse of the monitoring timer, a voltage of the smoothing capacitor, and determining that an open-contacts abnormality in the power source relay which is a failure due to open contacts of the power source relay exists, if the detected voltage of the smoothing capacitor is equal to or smaller than a predetermined voltage (Vt). the power source relay which opens and closes a connection between a direct current power source which drives an electric motor and a motor drive circuit which includes a plurality of switching elements and which supplies a drive current to the electric motor; a smoothing capacitor which is connected between a power source wire between the power source relay and the motor drive circuit and a ground wire to absorb ripples in current; a pre-charge circuit which opens and closes a connection between the direct current power source and the smoothing capacitor by bypassing the power source relay to pre-charge the smoothing capacitor; and a discharge circuit which is connected between the power source wire and the ground wire in parallel to the smoothing capacitor and which discharges electric charges stored in the smoothing capacitor via a discharge resistance when a discharge switch is closed, the power circuit abnormality detection method characterized by the steps of: switching on the pre-charge circuit to charge the smoothing capacitor to a voltage (Va) of the direct current power source; switching on the discharge circuit after charging the smoothing capacitor to the voltage (Va) of the direct current power source; switching on the power source relay; switching off the pre-charge circuit after switching on the power source relay, and starting a monitoring timer to count the discharging time of the smoothing capacitor; detecting, after elapse of the monitoring timer, a voltage of the smoothing capacitor, and determining that an open-contacts abnormality in the power source relay which is a failure due to open contacts of the power source relay exists, if the detected voltage of the smoothing capacitor is equal to or smaller than a predetermined voltage (Vt). switching on the pre-charge circuit to charge the smoothing capacitor to a voltage (Va) of the direct current power source; switching on the discharge circuit after charging the smoothing capacitor to the voltage (Va) of the direct current power source; switching on the power source relay; switching off the pre-charge circuit after switching on the power source relay, and starting a monitoring timer to count the discharging time of the smoothing capacitor; detecting, after elapse of the monitoring timer, a voltage of the smoothing capacitor, and determining that an open-contacts abnormality in the power source relay which is a failure due to open contacts of the power source relay exists, if the detected voltage of the smoothing capacitor is equal to or smaller than a predetermined voltage (Vt).