A CATV station apparatus subjects received optical signals to processes such as optical-electrical conversion, signal separation, coupling, and demodulation for obtaining an uplink signal. A received photocurrent monitoring section compares a received photocurrent at a optical receiving section with a predetermined reference current. If the received photocurrent is equal to or higher than the reference current, an amplifying section amplifies a signal output from a signal separating section at a predetermined level. If the received photocurrent is lower than the reference current, on the other hand, it is determined that a non-linear phenomenon, such as stimulated Brillouin scattering, has occurred in an optical fiber. The amplifying section then outputs a signal at a level which does not affect, even after being coupled with other signals, communications performed by other optical transmission systems.