The electronic control unit includes a control section monitoring a manipulation of an ignition switch of a vehicle and a timer section for automatically starting up said control section. The electronic control unit is provided with a fault diagnosis function of monitoring, by use of the control section, a state of an electric load connected to said electronic control unit and supplied with electric power from the vehicle battery when the main relay is in the on state after the control section outputs the stop command, and diagnosing whether or not the main relay is in a fault state where the main relay cannot be controlled from the on state to the off state on the basis of monitored state of the electric load.