A plurality of electronic control units (ECU) are classified under a plurality of ECU groups. A relay is provided for each ECU group, and connected between each ECU in a corresponding group and a power supply. Connection is established between respective ECUs by a communication network. When power is turned ON, each ECU outputs a flag indicating its own communication enabled state as long as there is no error in the communication function. A relay error detection portion detects, based on communication state information indicating whether each ECU is in a communication enabled state or communication disabled state, an error of the relay when the communication state information between ECU groups is inconsistent. Thus, an error in a relay that is provided for each group can be detected with a simple configuration.