Objective lens element

An objective lens element which has optically functional surfaces on an incident side and an exit side, which converges a first incident light beam of a wavelength λ1 through a base plate having a thickness t1 to form a spot, and which converges a second incident light beam of a wavelength λ2 longer than the wavelength λ1 through a base plate having a thickness t2 larger than the thickness t1 to form a spot, wherein

• at least either one of the optical function surfaces is a refractive surface which deflects the first and second incident light beams by refractive power all over the surface, the at least either one of the optical function surfaces being divided into an inner part which includes a rotational symmetry axis and through which the first and second incident light beams that substantially contribute to spot formation pass, and an outer part which is a ring-shaped region surrounding the inner part and through which only the first incident light beam that substantially contributes to spot formation passes, and the at least either one of the optical function surfaces having a plurality of discontinuous steps,
• each of the plurality of steps causes an optical path difference which is an integral multiple of the wavelength λ1, to the first incident light beam at a predetermined environmental temperature, and
• when an environmental temperature changes, tendency of spherical aberration change caused by change in shape of the refractive surface is opposite to tendency of spherical aberration change caused by change in optical path difference provided by the steps, the change in optical path difference being caused by based on change in refractive index.