PURPOSE: To miniaturize, to make the device light in weight and to shorten the access time of the automatic focusing device of an optical pickup.

CONSTITUTION: A diffusion lens group making the beam diameter of a transmitted laser beam 2a smaller than that of the laser beam 2a incident from an astigmatic lens group on the incident side of the laser beam 2a located nearer than the focusing position of the astigmatic lens group which causes the astigmatic aberration in the laser beam 2a made incident from an optical disk 1 is arranged in the astigmatic optical system 6 of an optical pickup. The power of the astigmatic lens group is made to be large and the power of the diverging lens group is made to be slightly smaller than that of the astigmatic lens group. Consequently, since the spacing between the astigmatic optical system 6 and a photodetector 8 is made to be small while keeping the numerical aperture of the astigmatic optical system 6 small, the optical pickup is miniaturized, made to be light in weight and the access time of the optical pickup is shortened.