Provided is a wavelength division multiplexer/demultiplexer having a flat wavelength response. In the wavelength division multiplexer/demultiplexer, a modified taper-shaped optical waveguide is interposed between an input waveguide and a first slab waveguide, such that the distribution of an optical signal input to an Arrayed Waveguide Grating (AWG) has a sinc-function shape. Thus, a flat wavelength response can be obtained in an output waveguide. In addition, the modified taper-shaped optical waveguide interposed to obtain a flat wavelength response has a small size and a simple structure, and thus can be applied to a conventional wavelength division multiplexer/demultiplexer without a design change.