PURPOSE: To make a fault detecting method usable in an environment with less protection such as a car by providing the method with a sensor means which issues the first signal which indicates that an electric circuit has a fault and the second signal which assumes a value corresponding to the occurring fault.

CONSTITUTION: A FAULT (the first signal) line transmits a signal which indicates that a fault occurred in a circuit 1 from an electric power circuit 1 to a microprocessor C. Both DIAGA and DIAGB (the second signal) lines transmit the information on diagnoses state, in short, the signal identifying a kind of fault detected, to the microprocessor C. A load or a voltage level which appears at output lines VOUT1 and VOUT2 connected to Darlington circuit pairs 2A and 2B, respectively, is sent to diagnoses modules 9A and 9B through lines 10A and 10B, respectively. Therefore, use in such bad environment as a car, etc., is possible.