An optical wavelength demultiplexer includes an optical lens, a first-side waveguide and a plurality of second-side waveguide. The lens has a wave number distribution-surface having a section where an angular frequency is constant, and said section having a shape which generally represents at least a part of an ellipse or a circle; a medium allowing group-velocity directions toward an inside of the section to allow said medium to exhibit a lens effect; a finite thickness defined by a distance between two parallel planarized surfaces of the medium; a first focusing point at an inside position of the lens; and a second focusing point at an outside position of the lens. The first-side waveguide is positioned for incidence of a wavelength-multiplexed optical signal into the lens. The plurality of second-side waveguide is so aligned that terminals thereof are positioned at respective focusing points of respectively different-wavelength components included in the wavelength-multiplexed optical signal.