A monitoring apparatus, based on an optical power monitored in photodetectors arranged at input and output ends of a transmission line, in a condition where pump light is supplied to the transmission line at a time of initial startup of an optical communication system, obtains a relationship for the noise light generated due to Raman amplification, between forward direction noise light power and backward direction noise light power, and while in service, converts the backward direction noise light power monitored by the photodetectors at the input ends of the transmission line into the forward direction noise light power in an arithmetic processing section, in accordance with the relationship obtained at the time of the initial startup. As a result the power of noise light generated due to Raman amplification, can be monitored in real time at high speed.