Any one of ECUs that are operating detects a present power supply voltage repeatedly. When the voltage detected at the present time is changed from the voltage detected at the previous time, a difference is calculated as a variation value and the repeatedly calculated variation values are integrated to provide a voltage variation integrated value. When the variation of the power supply voltage of an ECU becomes great relatively due to a loose connection in a power supply line, the calculated voltage variation integrated value becomes greater than those of the other ECUs. In addition, when a defect arises in a common power supply source, the voltage variation integrated value increases in all the ECUs. Therefore, each integrated value of the variation values of the power supply voltage in each of the ECUs enables an anomaly diagnosis of an occurrence point of the defect.