At least one power line is connected to a battery mounted at a vehicle. Plural ECUs are each connected to the at least one power line. A processor switches one ECU at a time of the plural ECUs from a second state to a first state by sending state switching signals to the plural ECUs. A power line to which plural ECUs are connected is a target power line. On the basis of current values of the target power line measured by a current measurement section when these plural ECUs are switched to the first state one at a time, the processor determines whether or not each ECU is in an abnormal condition.