The objective lens

PROBLEM TO BE SOLVED: To suppress a spherical aberration at a CD reproducing time without sacrificing the reproducing characteristic of a DVD by providing a specified ring-belt-like recessed part around an optical axis on the diffraction surface of an objective lens. SOLUTION: The objective lens is provided with a diffraction surface rotary symmetrical to an optical axis. A part of this diffraction surface is displaced along the optical axis as a ring-belt-like recessed part around the optical axis. Then, the ring-belt-like recessed part is the objective lens that an RMS aberration V satisfying an equation is provided with r1 and r2 becoming values in the vicinity of a minimum value when VdV/d(A21 )=0, dV/d(Δ1)=0, dV/dc=0. Where, (r) shows a radius from the optical axis, A63 shows a guinitic spherical aberration coefficient, A42 shows a tertiary spherical aberration coefficient, A21 shows a defocus aberration coefficient, (c) shows a wave front offset, Δ1 shows Δh/(n-1), (n) shows a refractive index of an objective lens material, r1 shows NAin /NA0 , r2 shows NAout /NA0 , NA0 , shows the diameter of a second numerical aperture, NAout shows the diameter of an are corresponding to an outside diameter numerical aperture and NAin shows the diameter of an area corresponding to an internal diameter numerical aperture.