To obtain a frequency multiplex transmission system that can receive a signal with best quality.

A light receiving circuit 11 receives an optical output signal from a node station to extract a data signal, a data processing section 12 converts the data signal into an electric signal, a switch 13 outputs a selection control signal that controls the selection of a required signal, and a multiplexer 14 outputs only an object electric data signal. A variable AMP 15 receives the electric data signal outputted from the data processing section 12 and the object electric signal outputted from the multiplexer 14 to control its gain. Thus, a degree of modulation of a transmission signal that is substantially in use can be increased by conducting the modulation with emphasis on a channel in use.