PURPOSE: To prevent the infiltration of a pulse noise with respect to other line by interposing a CR circuit between an input terminal and a photo transistor(TR) at a sender side to connect CR circuit and unsharpening steep the leading edge and the trailing edge of a pulse waveform.

CONSTITUTION: A transmission line 4 is formed by connecting a photo TR PC1, a constant current source 5 and a photo TR PC2 in a loop, and a CR circuit comprising a capacitor C and a resistor R2 is interposed between an inverter 2 and the photo TR PC1 at the sender side. The time constant of the CR circuit is changed to change the threshold level of an ON voltage of the photo TR PC1 at the sender side, thereby adjusting the leading and trailing time of the pulse waveform. Thus, the infiltration of pulse noise to other line is prevented.