An optical multiplexer/demultiplexer 1 having a plurality of first waveguides 3, a first slab waveguide 4, an arrayed waveguide grating 5 having a plurality of waveguides, a second slab waveguide 6 and a plurality of second waveguides 7, the individual waveguides being connected in the above order. The ratio of a distance between the plurality of first waveguides 3 at a connecting portion with the first slab waveguide 4 to a distance between the plurality of second waveguides 7 at a connecting portion with the second slab waveguide 6 differs from a ratio of a focal length of the first slab waveguide 4 to a focal length of the second slab waveguide 6.