Provided is a method in which a communication state between a main control unit and each sub control unit can be judged to be normal or abnormal, in a condition where a noise may be generated from a drive system power line. This method includes a power supply step s4, a test communication step (see arrows (3) and (4)), and a judgment step s6. In the power supply step s4, power is supplied to first sub control units 2a from a drive system power source 6 while no power is supplied to actuators 7 from the power source 6. In the test communication step, communication of a test signal is performed between the main control unit 1 and each of the sub control units 2. In the judgment step s6, whether the communication state is normal or abnormal is judged based on the result of the communication of the test signal between the main control unit 1 and each of the sub control units 2.