An electronic control unit has a gateway function for connecting a plurality of communication networks formed in a vehicle with each other and performs functions allotted beforehand by transmitting and receiving signals via the communication networks. The electronic control unit includes a plug-in connecting portion (39) to be plug-in connected with a relay unit (4) for converting protocols applied to the respective communication networks, thereby connecting the relay unit (4) with a communication bus (10), in the case of performing the gateway function, and a communication circuit portion (36) for transmitting and receiving signals between the communication bus (10) and the relay unit (4) when the relay unit (4) is plug-in connected with the plug-in connecting portion (39).