PURPOSE: To reduce the loss of parts for an optical circuit and to simplify the parts by controlling the polarization plane direction of plural incident light having different wavelength respectively which is made incident to a diffraction grating and dispersing said incident light by the diffraction grating.

CONSTITUTION: In a demultiplexer using a polarization plane maintaining fiber as a control means of the surface of polarization of incident light to the surface of the diffraction grating and a crystal rotor as a polarization plane separating means, 0.8μm band light λ1Wλn and 1.3μm band light λ'1Wλ'm transmitted through a polarization plane fiber 11 are transmitted through the crystal rotor 12 and then both light rays having respective wavelength bands are projected so that the planes of polarization are intersected at right angles. The light rays projected from the crystal rotor 12 are collimated by a lens 3 and made incident to a plate grating 14, and the incident light flux 15 is demultiplexed as projected light flux 16 and image-formed on fiber bundle 17. The intervals of respective fibers of the fiber bundle 17 are easily found out by determining each wavelength of light multiplied at its wavelength, the focal distance of the lens, the grating constant of the diffraction grating, and the incident angle.