The present invention relates to a wavelength division multiplexed (WDM) apparatus for monitoring optical frequencies, allowing optical channel frequency monitoring using the crossover property of an arrayed-waveguide grating and pilot tones that are in wide use in optical communication networks and enhancing efficiency of operation, maintenance, and management of WDM optical communication networks.The present invention provides a WDM apparatus for monitoring optical frequencies comprising optical demultiplexing means for optically demultiplexing optical signals including externally added pilot tone signals, photo-detecting means for photo-electrically converting the output of said optical demultiplexing means, and frequency estimating means for estimating the frequency of the optical signals by measuring the amplitudes of the pilot tones from the output of said photo-detecting means.