An arrayed waveguide grating type optical multiplexer/demultiplexer including an arrayed waveguide grating having a plurality of channel waveguides. Concave end faces of input-side and output-side slab waveguides are connected to opposite ends of the diffraction grating. At least one input waveguide and at least one output waveguide are connected to the other concave end faces of the input-side and output-side slab waveguides, respectively. The focal length of the output-side slab waveguide end face is longer than the focal length of the input-side slab waveguide end face. The optical multiplexer/demultiplexer has a sufficiently flat wavelength-dependent spectrum response in passing channel spacings, and helps construct an optical wavelength multiple communication system having a good signal-to-noise ratio.