To provide a multiplexer/demultiplexer that functions as an extremely small AWG optical filter in order to prepare a miniaturized and low-cost transceiver module necessary to realize a high speed transceiver for Ethernet (registered trademark). An optical multiplexer/demultiplexer includes: at least one input waveguide that has an input part at one end; a first slab waveguide of which one end is connected to the other end of the input waveguide; an arrayed waveguide group of which one end is connected to the other end of the first slab waveguide, the arrayed waveguide group having a plurality of waveguides; a second slab waveguide of which one end is connected to the other end of the arrayed waveguide group; and at least one output waveguide of which one ends are connected to the other end of the second slab waveguide and the other ends respectively have output parts, wherein each of the waveguides of the arrayed waveguide group has: a first bent part of which a waveguide extending direction changes 180 degrees or more; and a second bent part of which a waveguide extending direction changes 180 degrees or more in a direction opposite to a direction of the change of the first bent part.
