PURPOSE: To always control an objective lens to be in a just focusing state by fitting a focusing error detecting optical system to a member part for driving the objective lens and improving a focusing offset due to the difference of thickness between master disks.

CONSTITUTION: The acturator 19 of an objective lens 49 is connected to a lens holder 62 incorporating a focusing error detecting system. The recording beam 71 made incident from an objective lens 49 and the light beam 58 of a semiconductor laser 40 are reflected on a master disk 50 and only the light beam 58 of the laser 40 passes through a dichroie mirror 48 to be returned to the same optical path as an incidence by mirrors 46, 47 as a beam 59. The beam 59 is changed from a circular polarized light to an S-polarized light at a 1/4 wavelength plate 45 and then the optical path thereof is bent by 90° after being reflected on a polarized beam splitter 44 and further guided to two detecting systems by a non-polarized beam splitter 51. Detecting system consists of convex lenses 52, 55, columnar lenses 53, 56 and photodetectors 54, 57 and detects a focusing error by an astigmatic point to control the objective lens 49 to always be at a just focusing position.