A light demultiplexer in which a signal and a noise in each channel may be distinctly separated is provided. The light demultiplexer comprises a light-receiving element array for receiving light beams demultiplexed every wavelength from a wavelength multiplexed light beam and arranged in a straight line. The light-receiving element array includes a plurality of light-receiving elements for monitoring signals, and a plurality of light-receiving elements for monitoring noises. The light-receiving elements for monitoring signals and the light-receiving elements for monitoring noise are alternately arrayed in a straight line the direction thereof is the same as that of the arrangement of the demultiplexed light beams.