PURPOSE: To reduce performance deterioration due to an axis deviation between surfaces and the eccentricity of a lens carrier which are caused in lens machining by satisfying specific conditions as to the focal length, on-axis thickness, refractive index and the radii of curvature nearby the peaks of 1st and 2nd surfaces of a lens.

CONSTITUTION: A both-surface aspherical objective lens consisting of an aspherical surface with positive refractive power as its 1st surface and an aspherical surface with positive refracting power as its 2nd surface in order from a light source side satisfies inequalities IWIV, where F is the focal length of the lens, D the on-axis thickness, N the refractive index, and R1 and R2 the radii of curvature nearby the peaks of the 1st and 2nd surfaces. Consequently, this lens is used suitably as an infinite system objective lens for an optical disk, etc., and high power of β=-0.231 in image forming power, a large diameter of NA=0.45W0.50 in numerical aperture on a disk side, and optical performance close to the diffraction limit within a range of about 0.3W0.4mm are obtained.