When a computer of an ADK determines that a condition for performing self-diagnosis processing is met, or when performance of self-diagnosis processing is notified from a VP even if the condition for performing self-diagnosis processing is not met, the computer performs processing including: a step of performing self-diagnosis processing; a step of notifying the performance of the self-diagnosis processing to the VP; and a step of transmitting a result of diagnosis obtained through the self-diagnosis processing by the ADK to a central ECU when the self-diagnosis processing is completed.