To provide a diffractive optical element having a high light utilization efficiency, whereby light spots having a predetermined pattern can be stably formed, a projection device and a measuring device. The diffractive optical element of the present invention comprises a transparent substrate, a convexo-concave portion formed so as to be in contact with one surface of the transparent substrate and a filling portion with which concave portions of the convexo-concave portions are filled and which covers top surfaces of convex portions of the convexo-concave portion for planarizing the convexo-concave portion, wherein the convexo-concave portion has at least two stages on the surface of the transparent substrate; the top surfaces of the respective stages are parallel to one another; among the transparent substrate, the convexo-concave portion and the filling portion, the refractive indexes of at least the convexo-concave portion and the filling portion are different with respect to the incident light which enters from the normal direction of the surface of the transparent substrate; and the refractive indexes of the transparent substrate, the convexo-concave portion and the filling portion with respect to incident light are at most 2.2.

To provide a diffractive optical element having a high light utilization efficiency, whereby light spots having a predetermined pattern can be stably formed, a projection device and a measuring device.

The diffractive optical element of the present invention comprises a transparent substrate, a convexo-concave portion formed so as to be in contact with one surface of the transparent substrate and a filling portion with which concave portions of the convexo-concave portions are filled and which covers top surfaces of convex portions of the convexo-concave portion for planarizing the convexo-concave portion, wherein the convexo-concave portion has at least two stages on the surface of the transparent substrate; the top surfaces of the respective stages are parallel to one another; among the transparent substrate, the convexo-concave portion and the filling portion, the refractive indexes of at least the convexo-concave portion and the filling portion are different with respect to the incident light which enters from the normal direction of the surface of the transparent substrate; and the refractive indexes of the transparent substrate, the convexo-concave portion and the filling portion with respect to incident light are at most 2.2.

According to an embodiment, a recording/reproducing apparatus includes a diffraction grating and a light-receiving element. The diffraction grating divides return light from the guide layer in accordance with areas. The areas include a first area and a second area that does not overlap the first area. The light-receiving element includes (i) a first detecting cell group which receives a zero-order beam to which astigmatism is imparted, (ii) a second detecting cell group which receives at least one of a positive and negative first-order beam, which passes the first area and made astigmatic, and (iii) a third detecting cell group which receives at least one of a positive and negative first-order beam, which passes the second area and made astigmatic.