Each ECU transmits a frame to a frame generator. Then, the frame generator decomposes data contained in the received frame and then stores the data into a buffer memory, for each message type. The frame generator generates a message containing data to be transmitted to each ECU, and then transmits the message. Based on the state of a signal indicating the electric power supply status, the frame generator controls the permission or non-permission of transmission processing such that frame transmission is not performed to the ECU to which electric power supply from an electric power supply control device. When any communication part has received an abnormal frame, the frame generator does not include data contained in frames received afterward by the communication part, into a frame generated for other communication parts.