A microcomputer of an ECU which is a master determines whether to turn on or off the power supply of a slave ECU, and outputs a power supply control signal indicating power-on/off via serial communication on the basis of the result of determination. A signal superposition circuit accepts the output power supply control signal and transmits the accepted power supply control signal to a CAN bus to which a CAN transceiver is connected. In the slave ECU, a signal separation circuit individually receives a CAN communication signal and a serial communication signal transmitted to the CAN bus, while an input/output control circuit to which the serial communication signal is input as the power supply control signal outputs a signal to the power supply circuit to control power-on/off of the microcomputer.