A waveguide-type optical multiplexer/demultiplexer with low crosstalk characteristics and reduced wavelength dispersion is provided. A waveguide-type optical multiplexer/demultiplexer comprising, first, second and third optical multiplexer/demultiplexer circuits connected in multistage, each multiplexer/demultiplexer circuit has four input-output ports, third input-output port of first optical multiplexer/demultiplexer circuit is connected to third input-output port of second optical multiplexer/demultiplexer circuit, and fourth input-output port of first optical multiplexer/demultiplexer circuit is connected to fourth input-output port of third optical multiplexer/demultiplexer circuit, when wavelength division multiplex signals having wavelength λ1, λ2, λ3, λ4, λ5, λ6 . . . are input through first input-output port of first optical multiplexer/demultiplexer circuit, multiplex signals of odd wavelength λ1, λ3, λ5 . . . are output from first input-output port of second optical multiplexer/demultiplexer circuit, and multiplex signals of even wavelength λ2, λ4, λ6 . . . are output from first input-output port of third optical multiplexer/demultiplexer circuit.