PURPOSE: To reduce the weight of a moving optical system, and to shorten the access time, by dividing an optical system for executing the recording or reproduction to and from an optical disk, into a fixed optical system and a mobile optical system, and providing a focus driving member on the fixed optical system.

CONSTITUTION: A linearly polarized light of a second laser diode 69 for constituting a fixed optical system 60 is converted to a parallel luminous flux by its own collimator lens and reflected by a plane of polarization 64a of a polarized light beam splitter 64 through a half mirror 66, and emitted to a mobile optical system 80 side, as a circularly polarized light through a 1/4 wavelength plate 65. In such a way, by a built-in mirror 81 and an objective lens 82, a light beam is projected to the bit surface of an optical disk 100, and a reflected light is returned to the same path and allowed to reach a 1/4 wavelength plate 65, changed to a linearly polarized light therein and reflected by the plane of polarization 64a of the splitter 64, as a light beam being different by 90°, and allowed to reach a photodetector 70 through the half mirror 66. In this case, the detector 70 is divided into two in advance, its output voltage is made different from that of a detector 68 for projecting a light beam to the mirror 66 first, and a collimator lens 62 provided on a focus driving member 63 is moved.