Abstract: A relay diagnosis unit diagnosing a relay turning ON and OFF an electric power to a transmission control device. The transmission control device is equipped with a main control unit and a monitoring unit monitoring the main control unit on the basis of a signal from the main control unit, and is equipped with a relay diagnosis unit diagnosing a relay for turning ON and OFF an electric power supplied to the transmission control device and a monitoring unit diagnosis unit diagnosing the monitoring unit, and diagnosis is carried out at each of timing when a key position changes to an OFF position and at timing when the key position changes to an accessory position, so that both diagnoses, i.e., diagnosis for detecting malfunction of the relay and diagnosis for the monitoring function of the main control unit, can be carried out.