PURPOSE: To obtain simply a TTL output while keeping the effect to be exerted of other parts minimum value by adding a simple circuit to a current switching logical (ECL) circuit.

CONSTITUTION: A resistor R2, NPN transistors (TRs) Q3, Q4 and a constant current source I1 form a current switching logical circuit (ECL), and the switching operation is executed by a reference potential VREF set almost to a central value to a logical level of an input signal VIN. A resistor R1 and PNP TRs Q1, Q2 form a TTL output circuit and a TTL level is outputted depending on the ON/OFF of the TRQ3. Through such a constitution above, when the input VIN is at a high level, the input of the TTL circuit is kept to a high level. When the VIN is at a low level, a low level current flows to GND from the input of the TTL circuit via the resistor R1 and the potential is kept to the low level.