The optical element comprises a first diffraction structure for generating three beams by diffracting light from a light source and a second diffraction structure that exhibits structural birefringence for a prescribed polarized light returning to the light source. The first diffraction structure is formed on a first face of a translucent optical base material and the second diffraction structure is formed on a second face of the optical base material. The optical base material, the first diffraction structure and the second diffraction structure are formed of a single material.