A system for inverter control of a servo motor where in the inverter is connected to a velocity control section and a position detection section is connected to the servo motor. Primary and back-up power for operating the velocity control section and the position detection section is provided by power sources that are located in the velocity control section and are connected to the position detection section by cables. The cables used to provide signal transmission between the position detecting section and the velocity control section are switched by relays to transmit the power of a power failure back-up power supply at the time of a power failure. Also, a transmission signal transmitted from one of the position detecting section and the velocity control section to the other via a first cable connection is returned via a second cable connection and a connection fault may be judged depending on whether or not the return signal matches the transmission signal.