PURPOSE: To offer a countermeasure such as checking of an optical connector by raising an alarm as the deficient light receiving quantity when a prescribed amount or over of pulse signal attended with a high frequency noise is outputted at an output of an optical receiver after a count means is reset.

CONSTITUTION: If the luminous quantity of an optical signal inputted to the optical receiver 1 is deficient, an amplifier in the optical receiver 1 outputs a signal superimposed on the high frequency noise. When the level of the high frequency noise reaches nearly the same level as the reference level, lots of pulse signals are outputted in 1-bit time width in the output of the optical receiver 1 because of the high frequency noise. On the other hand, a counter 6 counts the pulse signal when the pulse signal attended with the high frequency noise is outputted from the optical receiver 1 while the clock pulse generated from a clock pulse generating circuit 4 is at an L level as shown in figure (c). When the counter 6 counts three pulse signals outputted from the optical receiver 1, an H level light receiving quantity deficiency detecting signal is outputted from its output terminal Y1.