When each slave ECU is powered on and activated while connected to a communication network through a harness, it reads out divided voltage potential applied by voltage dividing resistors in each ID determining signal line, and allows reception of a data packet transmitted from control ECU when a wait time corresponding to the divided voltage potential elapses. The control ECU successively transmits a data packet containing as a main body ID data to be allocated to each slave ECU, and each slave ECU sets ID data transmitted as its own ID.