A diffraction element having high diffraction efficiency and large wavelength-separation effect, which is excellent in mass-productivity and is formed by a simple process, is obtained. In a diffraction element having a rectangular grating formed in a surface of a transparent substrate whose cross-section is of an alternating recessed and projecting shape and each of the projecting portions is symmetrical, the period of the grating is equal to or smaller than the wavelength of incident light and the diffraction element is adopted so that the light is incident obliquely to its surface where the grating is formed.