To provide an array waveguide diffraction grating type optical multiplexer/demultiplexer which has good temperature dependence of optical characteristics.

The array waveguide diffraction grating type optical multiplexer/demultiplexer includes: an AWG chip which includes a first slab waveguide connected to a first input/output waveguide, an array waveguide comprising a plurality of channel waveguides connected to the first slab waveguide, a second slab waveguide connected to the array waveguide, and a plurality of second input/output waveguides connected to the second slab waveguide and is cut into two by a cut surface traversing the first or second slab waveguide; an underlying plate which is joined to a lower surface of the AWG chip and is cut in at least one part along the cut surface of the AWG chip; a compensation member which is provided like a bridge between areas of the underlying plate separated from each other by the cut surface and compensates for a temperature-dependent shift of a light transmission center wavelength of the AWG; and a warpage reduction member which has a linear expansion coefficient equal to or higher than that of the underlying plate and lower than that of the compensation member and is bonded to the compensation member via an adhesive so as to reduce stress between the underlying plate and the compensation member.