To provide a beam-condensing optical system and an optical pickup device that enable a plurality of types of optical information recording mediums to be recorded and reproduced.

An objective lens 3 is used in an optical pickup device for recording and reproducing information, a device that information can be recorded or reproduced on the optical information recording mediums 23, 24 having a transparent substrate of different thickness, by an optical flux from light sources 11, 12 of a different wavelength. The objective lens 3 consists of a first and second lenses 3a, 3b of positive refractive power which are arranged in order from the light source side; each lens is formed from a material with a specific gravity of 2.0 or below or a plastic material, and is provided with a loop-belt-like diffraction structure at least on one surface, the wavefront aberration is 0.07 λ1 rms or below with respect to the combination of a wavelength λ1, the thickness t1 of the transparent substrate, and an image-side numerical aperture NA1, and also the wavefront aberration is 0.07 λ2 rms or below with respect to the combination of wavelength λ2, the thickness t2 of the transparent substrate, and an image-side numerical aperture NA2. (assuming that t1<t2, λ1<λ2, and NA1≥NA2).