PURPOSE: To detect accurately interruption of an optical reception level by providing a divider inputting outputs of the 1st and 2nd peak detectors, operating the ratio of a signal component to a noise component and discriminating the interruption of the optical reception level when the value is smaller than a prescribed level.

CONSTITUTION: The output from an amplifier 2 includes the signal component and the noise component and only the signal component passes through a band pass filter 7 passing through only the band of the signal component and its peak value is detected by the peak detector 8. Further, the noise component is extracted by a band pass filter 9 passing through only the band of noise component having an out-band frequency of the signal component and its peak value is detected by the peak detector 10, and outputs VS, VN of the peak detectors 8, 10 are inputted respectively to the divider 11. When the optical input is sufficiently large, the level of the signal component is a value sufficiently larger than the level of the noise component, and when the optical input is small, the level of the signal component is sufficiently smaller than the level of the noise component. Then the divider 11 operates a value VS/VN and when the value is sufficiently small, it is discriminated surely the optical input level interruption.