To reduce the influence of a quantization noise at the time of A/D conversion without trouble in a function for controlling the transmitting signal strength.

A signal strength control circuit 140 controls the gain of a variable gain amplifier 120 into value capable of effectively using a bit resolution within a range not to exceed the allowable signal strength of the input signal of an A/D converting circuit 60 and also controls the signal strength of an up signal from a subscriber side modem 40. According to subscriber service signal strength information S20 from the signal strength control circuit 140, an output signal strength control part 145 controls the signal strength of a subscriber service signal equal to that at the time point of inputting to an input signal strength control part 135 of a remote node 10 by an output signal strength variable driver 156. Concerning a down signal from a terminal station 1 to the subscriber side modem 40, similar processing is performed as well.