PURPOSE: To resolve the problems of separately adjusting outgoing signals and incoming signals in order to adjust the input/output signal level of a line amplifier used for the transmission line of a bidirectional CATV facility and stopping transmitted signals by executing the adjustment while attaching/ detaching a specified circuit network.

CONSTITUTION: As variable functions, an adjuster 1 for low frequency and an adjuster 2 for high frequency are provided in a pseudo line circuit network 3, and a loss characteristic equal to the attenuating characteristic of a coaxial cable is arbitrarily continuously selected. Thus, the outgoing signals and incoming signals of the line amplifier in the bidirectional CATV facility can be adjusted without stopping those signals.