A curved surface diffraction grating fabrication method for fabricating a curved surface diffraction grating having a desired curvature with high accuracy, includes: a step of forming a diffraction grating pattern on a flat-shaped silicon substrate; a step of curving the silicon substrate on which the diffraction grating pattern is formed, by pressing, in a heated state, a fixing substrate having a shape with a desired curved surface onto the silicon substrate and of fixing the silicon substrate on which the diffraction grating pattern is formed to the fixing substrate having the shape with the curved surface, to fabricate a curved surface diffraction grating cast; and a step of bringing a member having flexibility into contact with the curved surface diffraction grating cast, to transfer the diffraction grating pattern to the member.