To enable a failure diagnostic device to detect abnormality in operation of an engine cooling fan.

An ECU 7 is arranged in an engine room, and a transistor 25 is turned on by a driving command signal to drive an engine cooling fan- motor 5. The internal temperature of the ECU 7 is detected by a temperature sensor 28. When the driving command signal is outputted to the engine cooling fan motor 5, based on the change in temperature of the ECU 7, detected by the temperature sensor 28 at the time when cooling air generated due to the driving of the engine cooling fan motor 5 strikes the ECU 7 to allow the ECU 7 to radiate heat, a microcomputer 20 judges the operation to be abnormal when the temperature difference between the temperature of the ECU 7 after starting the sending of driving command signals and the temperature of the ECU 7 when a specified time has elapsed after starting the transmission of driving command signals, is smaller than a specified value.