The present invention relates to a vehicle control device that is communicably connected to other control devices so that controller area network (CAN) communication can be performed through a communication bus and a diagnostic method thereof. The vehicle control device includes a CAN data transmission unit that can generate and transmit CAN data, a transmission stop unit that stops transmission of the CAN data transmitted from the CAN data transmission unit to the communication bus when a transmission stop signal is inputted, an abnormality detection unit that outputs the transmission stop signal to the transmission stop unit when detecting an abnormality in the vehicle control device, and a diagnostic unit that causes the abnormality detection unit to output the transmission stop signal regardless of the detection of abnormality, causes the CAN data transmission unit to transmit the CAN data, and diagnoses a transmission stop function that stops the transmission of the CAN data to the communication bus on the basis of a result of comparison between the transmitted CAN data and data on the communication bus.