Diffractive optical element and light irradiation device

A diffractive optical element and a light irradiation device which have high optical utilization efficiency, in which, even if the incidence angle of light deviates, the influence on diffracted light is small and desired diffracted light can be stably obtained, and which have little unevenness in diffracted light. A diffractive optical element is provided with a diffractive layer having in a sectional shape a high refractive index part in which a plurality of protruding portions are arranged side by side, and a low refractive index part and including a recessed section formed at least between the protruding portions. The protruding portion has a multistep shape provided with a plurality of step areas having different heights on at least one side of a cross-section thereof, and the cross-section of the protruding portion is at least partially provided with an inclined portion.