Diffractive optical element

Provided is a diffractive optical element capable of further reducing zero-order diffraction light. This diffractive optical element 10 comprises a diffraction layer 15 including: a high refractive index part 11 in which a plurality of projections 11a are arranged side by side in a cross-sectional shape; and a low refractive index part 14 that has a lower refractive index than the high refractive index part 11 and that includes a recess 12 formed at least between the projections 11a. The projections 11a have a sawtooth shape or a shape that imitates a sawtooth shape by a multi-stage outline shape. An inclined plane that is inclined with respect to a sheet surface of the diffractive optical element 10, which has a sawtooth shape or a sawtooth shape imitated by a multi-stage outline shape, has a concave curved plane that is concave toward the projections 11a.