A data transfer apparatus wherein a double switch system is adopted for safety. Two pieces of data inputted through a switch to a DI/DO control circuit (2) for peripheral devices are converted into serial data by circuits (Ar, Br), and the converted serial data are combined into a single piece of serial data by a circuit (Cr). The serial data is sent to a communication control section (1) of a CNC via a transmission line (3a). The section (1) divides the received serial data into two pieces of serial data, and converts each of the divided pieces of serial data into parallel data.
