PURPOSE: To increase noise margin and to attain high-speed operation without determining a level of a set input signal from the outside, by setting the amplitude of a logical input signal smaller than the amplitude of the set input signal.

CONSTITUTION: A current value i2 of a constant current source I2 connected to the 2nd ECL circuit 42 is determined suitably at first. A resistance value R3 of the 3rd resistor 53 is determined based on the current value i2 determined therewith. In determining suitably the current value i1 of a constant current source I1, resistors R1, R2 are selected suitably also. In limiting the range of level taken for a logical input signal A narrower than the range of level taken for a set input signal C in advance in this way, there is no need to introduce the set level from the outside for setting of the level of the set input signal C.