To simultaneously achieve monitoring of deterioration in an optical signal level, and also monitoring the amplitude of an optical signal in the usual using state, without especially stopping signal transmission and reception, in monitoring the quality of an optical transmission line.

A device includes: an optical-electric transducer which transduces an optical signal transmitted through an optical transmission line such as an optical fiber to an electric signal, and outputs the electric signal; and an amplitude monitoring unit which uses an output of this optical-electric transducer as an input, rectifies and smoothes the input, and outputs an alarm signal when the result of rectifying and smoothing the input is less than a threshold voltage.