In-vehicle units are on host vehicles including a subject vehicle and at least one nearby vehicle. Each in-vehicle unit transmits and receives information using inter-vehicle communication, acquires information indicating state of a control system controlling operation of each in-vehicle unit, generates an index data-item including an index value indicating state of the control system based on the acquired information, and transmits the generated index data-item using inter-vehicle communication. A subject in-vehicle unit in the subject vehicle includes: a communication processing section acquiring an index data-item from the nearby vehicle; a determination criterion specification section successively specifying a self-unit determination criterion being a determination criterion determining whether the control system in the subject in-vehicle unit operates normally, based on the index data-item; and a self-unit diagnosis test section determining whether the control system in the subject in-vehicle unit operates normally by comparing the self-unit determination criterion with the index data-item in the subject in-vehicle unit.