An optical demultiplexer having at least one Bragg diffraction grating for minimizing crosstalk on the sides of transmitting and receiving optical signals, as well as an optical communication module using the optical demultiplexer. The optical demultiplexer has first and second waveguides arranged adjacent each other in a predetermined section in order to perform mode coupling, so that the optical signals input through one end of the first waveguide are transmitted to a light receiving element through the second waveguide, and in which output light waves inputted through the other end of the first waveguide are output through the one end of the first waveguide. A first Bragg diffraction grating is formed on the second waveguide and haa wavelength selectivity, for minimizing crosstalk by transmitting a reception wavelength of the optical signals at about 100% and by reflecting a wavelength of the output light waves at about 100%.