An arrayed waveguide grating optical multiplexer/demultiplexer includes an arrayed waveguide grating formed on a substrate. The arrayed waveguide grating includes two slab waveguides, one arrayed waveguide, first input/output waveguides respectively connected to each end facet of slab waveguides, and second input/output waveguide-groups respectively connected to the end facets. The first input/output waveguides are provided for inputting or outputting a plurality of lights (.lamda.1.about..lamda.n) each of which has a different wavelength and multiplexed. The second input/output waveguides are provided for individually inputting or outputting a plurality of lights (.lamda.1.about..lamda.n). One arrayed waveguide 23 can be used to carry out simultaneously multiplexing and demultiplexing. It is able to carry out simultaneously multiplexing and demultiplexing with one arrayed waveguide grating.