To take a cable connection diagnosis of a remote PI/O not through communicating operation, but a simple system by switching remote PI/O's and performing communicating operation and taking a cable connection diagnosis of a remote PI/O which does not perform communicating operation.

A remote PI/O 4 completes a single communication with one remote PI/O by receiving process input data as an answer text on an RXD line of a communication port 6 through a transmission cable 2, a driver receiver 8, and a transmission/reception switching circuit 7, switches the transmission/ reception switching circuit 7 in order thereafter to perform transmitting/receiving operation up to a remote PI/O 5, and then returns to a communication with the remote PI/O 4. Further, a cable connection diagnosing circuit 10 is connected to a remote PI/O which does not perform communicating operation with a DO output signal of the communication port 6.