PURPOSE: To prevent the reflection due to the existence of an attenuation circuit from entering a transmission signal and to eliminate the high frequency noise at the time of switching from the transmission signal.

CONSTITUTION: As a signal transmission circuit BD, the circuit having the output impedance which is equal to the characteristic impedance specific to a transmission line pair TM is applied. As an attenuation circuit AT, the circuit composed of balance four-terminal network circuits R11 to R15 and C1 where the input impedance and the output impedance are equal to the characteristic impedance specific to the transmission line pair is applied. As the attenuation circuit AT, the circuit having the high-pass cut-off characteristic according to the speed of the transmission signal is applied.