PURPOSE: To reduce the size and cost of a device by constituting such a two-way type digital input/output circuit that a digital signal is inputted and outputted by one circuit in common.

CONSTITUTION: When the circuit functions as an input circuit, an input contact 3 is connected between terminals T1 and T2; and a specific signal is supplied to a switching circuit 13 to select a high frequency signal oscillator 14 and a specific signal is supplied to a terminal T4 to prevent an output transistor 11 from turning on (off). When the circuit functions as an output circuit, a load 2 is connected between the terminals T1 and T2 together with a power source 1 for the output operation and a specific signal is supplied to the switching circuit 13 through a terminal T3 to select a low frequency signal oscillator 15.