PURPOSE: To prevent a service data from being demodulated/modulated erroneously by a service data demodulation modulation circuit (SD DEM) by disconnecting an output of the SD DEM if the optical input level goes to a very small level.

CONSTITUTION: An output of an HV AGC CONT 6 is fed to the SD DEM 10 to interrupt the output of the SD DEM based on the output. The SD DEM output is zero when the output of the HV AGC CONT maximizes the gain and coincident with when the optical input level is minimized. Thus, it is prevented that a service data is demodulated/modulated by the SD DEM as in the presence of service data by noise caused from each section of an optical reception circuit in the event of the optical input level being a very small value.