A vehicle monitoring system which can readily identify information transmitted from each detecting device without requiring registration of identifying information of the detecting device is to be provided. Detecting devices transmit vehicle information including at least the result of detection and self-identifying information; a tractor relaying device and a trailer relaying device add their self-identifying information to vehicle data received on the basis of the identifying information of the detecting devices and transmit the vehicle data, and transmit registration data including their own relaying device IDs and types indicating the registration of the relaying device IDs; the tractor relaying device stores the relaying device ID of the registration data transmitted from the trailer relaying device on the basis of the type information and transmits that registration data, and transmits vehicle data received from the trailer relaying device on the basis of the relaying device ID. The display device receives the registration data and the vehicle data transmitted from the tractor relaying device, and stores the relaying device ID of the registration data on the basis of type information.