Patent ID: 7340165

Claim:
An optical amplifying device disposed on a transmission path of a wavelength division multiplexing (WDM) signal, comprising: an optical amplifier amplifying a WDM signal; a detecting unit detecting a change in a transmission wavelength count contained in the WDM signal and/or a change in light receiving level of the WDM signal received from a transmission path and inputted to said optical amplifier; a measuring unit measuring an optical signal to noise ratio of the WDM signal outputted from said optical amplifier; an update unit updating a reference value for evaluating the measurement value of the optical signal to noise ratio obtained by said measuring unit when said detecting unit detects the change; a judging unit judging whether or not the measurement value deviates from an allowable range based on a reference value; and an output unit outputting an error of said optical amplifier if the measurement value deviates from the allowable range.