Abstract: The optical performance monitoring apparatus includes a first optical distributor for distributing a WDM optical signal branched from an optical transmission line, a plurality of wavelength selectors, each for selecting a predetermined wavelength optical signal from the optical signal distributed, a plurality of first optical detectors, each for detecting power of the predetermined wavelength optical signal for a corresponding one of channels selected by the wavelength selectors; and a signal processor for measuring the power for each channel of the WDM optical signal, a total ASE noise power, and an optical signal-to-noise ratio for each channel from the digital value.