One side surface of a diffraction grating is securely held by a holder, and further, the holder is securely supported on a substrate by a supporter in such a manner that the diffraction grating held by the holder is not brought into contact with the substrate. In this manner, the diffraction grating is fixed only onto one side surface to the supporter via the holder, and further, the diffraction grating is not brought into contact with the substrate. Therefore, the lower portion of the supporter is displaced by heat generated in the substrate, however, the displacement of the supporter cannot adversely influence directly on the diffraction grating since the holder is interposed between the displaced portion and the diffraction grating.