Patent ID: 8406110

Claim:
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 λ 1 through a base plate having a thickness t 1 to form a spot, and which converges a second incident light beam of a wavelength λ 2 longer than the wavelength λ 1 through a base plate having a thickness t 2 larger than the thickness t 1 to form a spot, wherein at least either one of the optically functional surfaces on the incident side and the exit side includes a diffraction portion which satisfies at least either one of the following formulas (1) and (2): θ 1 ×θ 2 <0(“×” represents multiplication) (1), and (sin θ 2 )/λ 2 =−(sin θ 1 )/λ 1 (2), where θ 1 is the diffraction angle of a light beam having a diffraction order providing the maximum diffraction efficiency at the wavelength λ 1 (θ 1 ≠0), and θ 2 is the diffraction angle of a light beam having a diffraction order providing the maximum diffraction efficiency at the wavelength λ 2 (θ 2 ≠0).