An in-vehicle unit, which is used in each of host vehicles including a subject vehicle and nearby vehicles, includes an inter-vehicle communicator to perform inter-vehicle communication. The in-vehicle unit in the subject vehicle includes a criterion value setting section and a different-unit anomaly detection section. The criterion value setting section receives a different-unit installation state index via the inter-vehicle communicator and sets successively a criterion value of an installation state index from the received different-unit installation state index specified based on an output value from a sensor instrument that successively outputs an output value corresponding to an installation state of the in-vehicle unit on each nearby vehicle. The different-unit anomaly detection section detects an abnormal installation state of the in-vehicle unit on a target nearby vehicle by comparing the criterion value with the different-unit installation state index received from the target nearby vehicle.