Each of controllers includes: a transistor; a data processor for executing predetermined data processing and turning the transistor on or off in accordance with the data processing result; a pull-up resistor connected to an output terminal of the transistor; and a level separating resistor connected to the output terminal of the transistor, and the data processor of each controller detects a signal level of a control signal output from the output terminal of the transistor and judges whether or not any defect has occurred in its own controller, based on the detection result.