PURPOSE: To incorporate a self-loop-back function capable of looping back output signals from an output element as input signals to an input element in the inside.

CONSTITUTION: The output element 2, the input element 3, a selection element 6 for loop back and a discrimination circuit 9 are incorporated in this circuit 70 for input/output signal control, and when the selection element 6 for the loop back is defined as valid by control signals for the loop back from the discrimination circuit 9, the output signals 103 from the output element 2 to an external equipment are inputted as the input signals 104 of the input element 3. Thus, at the time of inspecting the external input/output function of the circuit for the input/output signal control in a manufacture stage or in a site, the need of performing wiring so as to turn the output signals to the input signals on the outside is eliminated and a loop back test is facilitated.