PURPOSE: To read out respective wavelength components by optical sensors by receiving an optical beam consisting of plural wavelength components, decomposing the received beams into respective components and individually projecting the components.

CONSTITUTION: One plane of a Fabry-Perot etalon is deviated from a parallel state so as to be slightly inclined and an one-dimensional optical sensor array is arranged along the inclination. An etalon array obtained by combining the parallel Fabry-Perot etalon and an optical wedge or step-wisely increasing or reducing a face interval may be used instead of the inclined Fabry-Perot etalon. When an optical beam 25 is made incident upon the inclined Fabry-Perot etalon, the wavelength of transmitted light or reflected light is changed along the inclination because the face interval is changed in accordance with an incident position, and the wavelength of an outgoing optical beam is also increased to the increasing direction of the face interval. Since incident light is projected from a different position along the inclined direction of the etalon in each wavelength component, each wavelength can be read out by the one-dimensional optical sensor array. Other constitution is also the same in principle.