A housing 20 is equipped for supporting, from a side, a platform of a wavelength selection device comprising an input/output port 10, a collimator 11, an expanding optical system 12, a spectroscopic element, a collecting optical system 14 and a micro electro mechanical system (MEMS) mirror array 15. Because the above noted optical member is supported from the side only, influences of a thermal expansion is limited to the height direction of the optical member and the optical axis direction. By these aspects, the influence of thermal expansion is limited to a two-dimensional from a common three-dimensional, thereby enabling a design of a countermeasure to an influence of a thermal expansion. Also, the support from the side does not create a dead space thereby making the wavelength selection device compact.