A multiplexer/demultiplexer includes: a waveguide chip including a first chip and a second chip that are divided by a plane and obtained by cutting, together with a substrate, in a direction crossing an optical axis, a first slab waveguide of an AWG including the first slab waveguide and a second slab waveguide that are formed on the substrate; a first base to which the first chip is fixed; a second base separated from the first base and to which the second chip is fixed; and a member that has one end fixed to the first base or chip and another end fixed to the second base or chip, in a state in which cut surfaces of the first and second chips face each other, and that is configured to move the first base and the second base relatively to each other along the plane by expanding/contracting when temperature changes.