The present invention relates to a wavelength demultiplexer. More particularly, the present invention provides the wavelength demultiplexer with straight optical waveguide that minimizes the bending loss of optical waveguide caused in the wavelength demultiplexer. A wavelength demultiplexer with straight optical waveguide in accordance with the present invention comprises an optical power distributor, a plurality of optical waveguides, and an optical power combiner. The optical power distributor evenly divides multiplexed input light by intensity. The number of optical waveguide transmits the divided multiplexed light and causes constant optical path length differences among adjacent waveguides. The optical waveguide is straight optical waveguide and includes two parts of different effective refractive indices. The optical power combiner receives output signals of the plurality of optical waveguides and separates the output signals by phase.