To converge diffracted light of sufficient light quantity to the information recording surface of an optical information recording medium by performing mode hop correction and improving diffraction efficiency even when surface sagging is formed on a diffraction structure part of an objective lens and a change in wavelength is generated in a beam of short wavelength projected from a light source.

The diffraction ring structure part formed on the incident surface 161 of a light beam L of reference wavelength λ0 (380[nm] ≤ λ0 ≤ 450[nm]) is constituted so that the diffraction degree K of diffracted light whose diffraction efficiency is maximum is set to 3 ≤ K ≤ 14 (K is an integer). Even when surface sagging is generated in the diffraction ring structure part and a change in wavelength is generated in the light beam L, the objective lens 16 having a mode hop correcting function improves the diffraction efficiency of K-th degree diffracted light and condenses the light beam of sufficient light quantity to the information recording surface 22 of an HD-DVD 20 which is the information recording medium to perform either one or both of the reproducing and recording of information.