To provide a diffraction optical element and a method of manufacturing the element by which an antireflection film can be continuously formed with uniform film thickness on the surface of a periodical grating having a grating consisting of a plurality of recesses and projections.

In the diffraction optical element 1, a first antireflection film 41 is formed on the surface of a periodical grating 2 by a dipping method including processes of immersing a glass substrate 3 on which the periodical grating 2 is formed in a vessel 51 reserving a liquid 40 of the antireflection material, drawing the substrate up and drying by blowing hot air F. The film thickness can be controlled by the viscosity of the liquid, drawing speed and drying speed. Since the periodical grating 2 is drawn while the forming direction of the grating is always perpendicular to the liquid surface 43, an excess antireflection material flows and drops along the periodical grating 2 without leaving the antireflection material to remain in the grating. After drying the antireflection material remaining on the substrate surface, the first antireflection film 41 can be continuously formed with uniform film thickness. At the same time, a second antireflection film 42 is also formed on the back face 32 of the glass substrate 3.