PURPOSE: To facilitate an easy two-way transmission and thus to simplify the transmission/reception circuit by supplying the control signal independently from the logic signal of the input signal and then setting simultaneously the two types of output voltage leves to low levels via the control signal.

CONSTITUTION: The two-way transmission circuit is formed by transmission/reception parts 1 and 2 of emitter-coupled logic circuit ECL which uses the binary signal featuring the high and low states of the voltage level and also delivers the signal identical logically to the input signal and the signal reverse logically to the input signal. Then control signals are supplied, independently from the input logic signals, to input-side drivers 11 and 21 of parts 1 and 2. And two types of output terminal levels delivered to receivers 12 and 13 as well as 22 and 23 of thereception side are set forcedly to low levels via drivers 11 and 21. As a result, the two-way transmission can be facilitated at parts 1 and 2, thus simplifying the structure for parts 1 and 2.