An optical wavelength multiplexer/demultiplexer configured to demultiplex a multiplexed optical signal with a channel number N and a channel frequency interval Δf into optical signals each with a wavelength λ, the optical wavelength multiplexer/demultiplexer comprising: an input waveguide configured to branch an input multiplexed optical signal into two inputs and to output the inputs from two waveguides (12, 13); a first slab waveguide including a first end connected to second ends of the two waveguides (12, 13); an arrayed waveguide connected to a second end of the first slab waveguide and including a plurality of waveguides provided at predetermined intervals and having different waveguide lengths; a second slab waveguide including a first end connected to the arrayed waveguide; and an output waveguide including two waveguide groups (51, 52) each including a plurality of waveguides connected to a second end of the second slab waveguide, wherein one of the two waveguides (12, 13) is connected to a position disposed circumferentially outward by a predetermined clearance interval ΔW GHz from a central position on a connection end surface of the first slab waveguide and the other waveguide is connected to a position displaced circumferentially inward by a predetermined interval (ΔW + Δf) GHz from the central position on the connection end of the first slab waveguide 20, the two waveguides (12, 13) are arranged such that, in a case where a connection position where one of the two waveguides is connected to the first slab waveguide is set closer to the other waveguide by a channel frequency interval Δf, a central position between the two waveguides aligns with a central position on a connection end surface of the first slab waveguide, and the two waveguide groups of the output waveguide are arranged such that a central position between the two waveguide groups aligns with a central position of the second slab waveguide, and an interval between a connection position where the other waveguide is connected to the first slab waveguide and the central position on the connection end surface of the first slab waveguide is set equal to an interval between a connection position where the waveguide groups are connected to the second slab waveguide and a central position on a connection end surface of the second slab waveguide, and an interval between adjacent waveguides in each of the two waveguide groups is set double the channel frequency interval. an input waveguide configured to branch an input multiplexed optical signal into two inputs and to output the inputs from two waveguides (12, 13); a first slab waveguide including a first end connected to second ends of the two waveguides (12, 13); an arrayed waveguide connected to a second end of the first slab waveguide and including a plurality of waveguides provided at predetermined intervals and having different waveguide lengths; a second slab waveguide including a first end connected to the arrayed waveguide; and an output waveguide including two waveguide groups (51, 52) each including a plurality of waveguides connected to a second end of the second slab waveguide, wherein one of the two waveguides (12, 13) is connected to a position disposed circumferentially outward by a predetermined clearance interval ΔW GHz from a central position on a connection end surface of the first slab waveguide and the other waveguide is connected to a position displaced circumferentially inward by a predetermined interval (ΔW + Δf) GHz from the central position on the connection end of the first slab waveguide 20, the two waveguides (12, 13) are arranged such that, in a case where a connection position where one of the two waveguides is connected to the first slab waveguide is set closer to the other waveguide by a channel frequency interval Δf, a central position between the two waveguides aligns with a central position on a connection end surface of the first slab waveguide, and the two waveguide groups of the output waveguide are arranged such that a central position between the two waveguide groups aligns with a central position of the second slab waveguide, and an interval between a connection position where the other waveguide is connected to the first slab waveguide and the central position on the connection end surface of the first slab waveguide is set equal to an interval between a connection position where the waveguide groups are connected to the second slab waveguide and a central position on a connection end surface of the second slab waveguide, and an interval between adjacent waveguides in each of the two waveguide groups is set double the channel frequency interval.