To accurately machine an original die for forming Fresnel lenses having an ideal blaze-grating.

The path and the feed quantity of a diamond bit 6 in relation to a blank are controlled so that an edge 6b of the diamond bit 6 is moved toward the center 0 of rotation the blank while it traces a target two dimensional contour line 30. Besides, the target two dimensional contour line is referred to a contour line of the crosssectional shape of an ideal blaze-grating. Since the edge 6b of the diamond bit 6 has roundness having the diameter (3μm or more and 10μm or less) determined by the feed rate, the spherical shape of the edge 6b of the diamond bit 6 is transferred to the surface of the blank as the blank A is rotated. Finally, a locus formed by rotating the target two dimensional contour lone 30 around the center O of rotation, namely an inverted shape of the ideal blaze-grating is created.