An optical multi/demultiplexer is provided that has a wide passband and small crosstalk among its all channels. It includes first and second arrayed waveguide gratings and a 2×2 optical signal processor. The 2×2 optical signal processor includes first to fourth directional couplers, and first to third delay lines. The first to third delay lines have thin film heater phase shifters for correcting phase errors involved in fabrication. Utilizing the 2×2 optical signal processor having a transmission spectrum with a square profile having a wide passband and rejection band makes it possible for the optical multi/demultiplexer that combines the arrayed waveguide gratings with the waveguide type circulating filter (2×2 optical signal processor) to widen the extinction bandwidth of the adjacent channel.