PURPOSE: To ensure the stable 3-state control with simple circuit constitution by setting a non-inverted logic circuit between two inverted logic circuits and setting a high impedance after an output is set at a high level.

CONSTITUTION: Both inverted circuits 1 and 2 have the same circuit constitution. A signal B is supplied to the circuit 2 and the output terminal 2-4 of the circuit 2 is set at '0'. Under such conditions, the 3-state control signal A of the circuit 1 is set at '1' and therefore the output terminal 1-4 of the circuit 1 is set at '0'. Then a current flows to a diode D3. As a result, the terminal 2-4 is set at '1'. At the same time, '0' of the terminal 1-4 is supplied to a non-inverted logic circuit 3 and the terminal 3-4 of the circuit 3 is set at '1' to cut off the current flowing to a diode D6. Thus the terminal 2-4 has a high impedance.