To effectively eliminate a warp of a substrate in manufacturing a diffraction optical element.

The method of manufacturing the diffraction optical element which has a step to stick and fix an organic material film 3 to a transparent substrate 1A, a step to form a plurality of diffraction gratings RL based on rectangular projections and recessions on the organic material film 3 in a grating array shape and a step to individually separate a plurality of the formed diffraction gratings, and also includes a step to form grooves, to alleviate the warp on the substrate on which the warp is generated by thermal contraction of the organic material film 3, with at least one line for both vertical and horizontal directions of the grating array shape of the diffraction gratings from the organic material film 3 side with depths larger than the thickness of the organic material film 3 and without damaging the strength of the transparent substrate 1A.