An in-vehicle unit used in each of host vehicles including a subject vehicle and nearby vehicles communicates information using inter-vehicle communication, generates an index data-item including an index value indicating an operational situation of a sensor instrument based on a detection result from the sensor instrument, and transmits the generated index data-item via the wireless communicator. A subject in-vehicle unit used in the subject vehicle includes: a communication processing section that acquires the index data-item from nearby in-vehicle units; a determination criterion specification section successively specifies a different-unit determination criterion based on the acquired index data-item; and a different-unit diagnosis test section that determines whether a sensor instrument in a target nearby in-vehicle unit operates normally, by comparing the different-unit determination criterion with the index data-item acquired from the target nearby in-vehicle unit.