To provide a fault diagnoser which can prevent lock conditions of a wiper, determining the abnormality of the wiper, and can perform the determination of the lock condition more accurately.

In a fault diagnosing unit which is equipped with a thermal FETQA for controlling the power supply from a power source to a wiper motor 102, being switched, according to a control signal, a comparator CMP2 compares the load current flowing in the wiper motor 102 with a prescribed threshold. A cycle counter 107 detects the operation cycle of the wiper motor 102 from the result of comparison through the change in the passage of time of the comparator CMP2, and a cycle determining circuit 108 judges the abnormality of the wiper 102, based on the operation cycle detected with the cycle counter 107. Then, a PWM controller 109 changes the duty ratio of the PWM control signal, based on the output from the cycle determining circuit 108, or stop the drive of the wiper motor.