PURPOSE: To prevent the abnormal operation of machine tools and to improve safety by emitting a dummy command signal when control data is not applied on a transmitter side and judging a trouble and executing processing if there is no input of the data or the dummy command signal on a receiver side.

CONSTITUTION: The control data is transmitted to a control box 3 by a serial transmission means upon operation of a CPU in an operation box 1. The dummy command signal is transmitted to the control box at every prescribed time when there is no control data for the prescribed period or above. The data or dummy command signal is received by the serial reception means in the control box 3 and is arithmetically processed by the CPU, by which motors 5, 7, 9 are controlled. A no-input discriminating means operates and a trouble- shooting means emmergently stops the respective motors when the serial reception means does not receive any signal for the prescribed time or above.