To improve the duty ratio of a reception digital signal by regulating the pulse duration of the reception digital signal.

When the level of an input signal at an input terminal IN is higher than a prescribed value L1 decided by variable resistors VR1 and VR2, a transistor Q is turned on but when that level is lower than a prescribed level L2, the transistor Q is turned off. When prolonging the duration of an output pulse signal from an output terminal OUT, the level L1 is set low so as to turn on a transistor Q1 by regulating the values of resistors VR1 and VR2 or the level L2 is set low so as to turn off the transistor Q by regulating the number of diodes D1-D3 connected between the input terminal IN and a base B of the transistor Q1. When shortening the duration, reverse control is performed. Namely, the rising timing of a pulse signal is regulated by changing the values of the resistors VR1 and VR2 or the falling timing is regulated by changing the number of diodes, and the duty ratio is regulated so that the duration of a regenerative digital signal can be equal to the duration of a transmissive digital signal on the transmission side.