In an incoming path, light fed to an input port is demultiplexed by a wavelength demultiplexer in terms of wavelength, converged by a first light-converging optical system, reflected by a first reflecting mirror, turned into parallel light by the first light-converging optical system, and multiplexed by the wavelength demultiplexer, so as to be fed to a reflecting surface of a rectangular reflecting mirror. In an outgoing path, the light reflected by the rectangular reflecting mirror is demlutiplexed by the wavelength demultiplexer in terms of wavelengths, converged by the first light-converging optical system, reflected by the first reflecting mirror, turned into parallel light by the first light-converging optical system, and multiplexed by the wavelength demultiplexer, so as to be fed to an output port. An image of each wavelength light component reflected by the first reflecting mirror is inverted between the incoming and outgoing paths.