In a spectroscopic module 1, a flange 7 is formed integrally with a diffraction layer 6 along a periphery thereof so as to become thicker than the diffraction layer 6. As a consequence, at the time of releasing a master mold used for forming the diffraction layer 6 and flange 7, the diffraction layer 6 formed along a convex curved surface 3a of a main unit 3 can be prevented from peeling off from the curved surface 3a together with the master mold. A diffraction grating pattern 9 is formed so as to be eccentric with respect to the center of the diffraction layer 6 toward a predetermined side. Therefore, releasing the mold earlier from the opposite side of the diffraction layer 6 than the predetermined side thereof can prevent the diffraction layer 6 from peeling off and the diffraction grating pattern 9 from being damaged.