A plurality of electronic control units is continuously supplied with electric power to be capable of communication one another even when respective controlled devices are in inoperative condition. Each ECU transmits a sleep NG information along with its device ID when it not allowed to remain in the sleep mode. An abnormality detection unit connected to the ECUs through communication lines monitors the sleep NG information of each ECU to detect the ECU remaining activated for more than a wait period which is required for the ECU to shift to the sleep mode. The device ID of the ECU which is detected as remaining activated is stored in a memory so that any abnormal location may be specified by the stored device ID.