An apparatus for detecting a fault of a flying capacitor of an insulated condition detecting unit measures (step S1), at the start of discharge of the flying capacitor, a discharge voltage of the flying capacitor according to a charge voltage of a read capacitor. After a period from the start of discharge, the apparatus measures (step S3) a discharge voltage of the flying capacitor according to a charge voltage of the read capacitor. According to a difference between the measured discharge voltages, the apparatus obtains (step S5) a discharge amount of the flying capacitor, compares the discharge amount with a reference threshold, and diagnoses (step S7) an open fault of four capacitors that constitute the flying capacitor.