PURPOSE: To efficiently branch the rays of different wavelength bands and to increase the multiplicity by installing a multiplex interference film filter before diffraction with a diffraction grating, and branching waves at every wavelength band.

CONSTITUTION: If optical fibers 10W20 arrayed in accordance with the number of wavelengths and a transmission optical fiber 1 are placed on the focal plane of a lens, the rays of different wavelengths λ11Wλ20 propagating in the fiber 1 are emitted from the fiber 1, after which they are made into parallel rays by a lens 2, and these parallel rays are made incident to a multiplex interference film filter 3. The wavelengths λ11Wλ15 and λ16Wλ20 are in different wavelength bands, and if the filter 3 which allows the transmission of the rays of the wavelengths λ11Wλ15 and reflects the rays of the wavelengths λ16Wλ20 is selected, the rays of the wavelengths λ11Wλ15 transmit through the filter 3 and are made incident to a reflection type diffraction grating 4. Thereby, the rays of different wavelength bands are branched efficiently and the multiplicity is increased.