A reference signal is transmitted from one of a plurality of robot control devices connected by communication connecting device to the other robot control device. The timing of generation of an operation basic period signal in each of the other robot control devices is synchronized with the timing of generation of an operation basic period signal in the one of the plurality of robot control devices, based on a time interval from generation of the operation basic period signal until transmission of the reference signal in the one of the plurality of robot control devices, a time interval from generation of the operation basic period signal until reception of the reference signal in each of the other robot control devices, and a communication delay time required for communication between the one of the plurality of robot control devices and each of the other robot control devices.