PURPOSE: To multiplex or demultiplex rays consisting of three or more kinds of wave, by arranging a filter group, which reflects the light having a specific wavelength and permits the light to pass through, on one end face of a distributed index lens and providing a reflective mirror.

CONSTITUTION: Filter films 9 and a reflective mirror 10 are formed on joint surfaces of transparent materials 8 which consist of glass or the like and have a wedge-shaped section so that these filter films and the reflective mirror form different angles to th end face of a lens 4. Filter films 9 reflect rays having specific wavelengths different from one another. The light having a wavelength λi incident from an optical fiber 1-1 for incidence passes through the lens 4 and is relfected by the filter film 9-i and is coupled to an optical fiber (1-(i+1)) for emission and is propagated, and rays having many wavelengths are branched. The light of a wavelength n is reflected by the reflective mirror 10. It is possible that rays having different wavelengths are made incident from the optical fiber for emission and their multiplexed waves are taken out from the optical fiber for incidence.