A wavelength demultiplexer is equipped with a spectroscopic means (which separates light that is input from multiple input light paths, and outputs the light to multiple output light paths) and a light path switching device (a device that switches the light paths that are input to the spectroscopic means, with the switching being performed by an external operation), and the light path switching device may be a device that distributes the input from one input port to multiple output ports. The light path switching device and the spectroscopic means are polarization-independent, with the input light paths, the output light paths, and the light paths between the light path switching device and the spectroscopic means being polarization-maintaining light paths, so the relative polarization configuration is the same for the input light and the output light.