PURPOSE: To improve the demultiplexing efficiency with a simple constitution and to reduce influences upon the other wavelength components by prescribing widths of single mode optical wavegudes of both channels coupled to each other in the position where they are parallel with and close to each other and providing a grating only one optical waveguide.

CONSTITUTION: When single mode optical waveguides 1 and 2 of the first and the second channels which have core widths a1 and a2 different from each other are coupoed in a coupling part 3 where they are parallel with and close to each other, the light having a prescribed wavelength which is made incident on the waveguide 1 is branched through the coupling part 3 and goes backward on the waveguide 2 and exits. The efficiency of taking-out of the branched light is maximized if widths a1 and a2 are set to satisfy 1.2a2≤a1≤3a2, and harmful coupling is suppressed to eliminate crosstalk if a grating 6 having a prescribed frequency in the light propagation direction is provided only in one waveguide 2, and thus, the demultiplexing efficiency is improved with a simple constitution, and the light is demultiplexed with less influences upon the other wavelength components.