The monitoring of the qualities of the optical signal transmitted from a light amplifying relay and nodes of transmission networks. An object of the present invention is to provide an apparatus and method for correct monitoring the qualities of the optical signal. While conventional methods monitor the qualities of the optical signal solely by the intensity of the optical signal, the present invention provides an apparatus and method for extracting the clock component of data from the optical signal transmitted and measuring the magnitude and then detecting the presence or absence of the signal and the error rate upon receiving the deteriorated signal due to the chromatic dispersion. The present invention comprises the simple measuring apparatus having a optical coupler 1, a signal conversion means 2, a narrow optical detector 3, a super high frequency rectifier 4 and a voltage detector 5. In application of management and maintenance of optical transmission systems using a light amplifier and photoelectric transmission networks, the present invention provides a high reliability in monitoring the qualities of the optical signal.