PURPOSE: To realize the stable detection for both the out-of-focus and a shifted tracking at one time, by detecting the shifting direction of a beam spot on a photodetector which is formed by extracting the light asymmetrical to the optical axis of the light beam that is reflected from an information forming layer and passed through an objective lens.

CONSTITUTION: The laser light 11 is condensed on an information forming layer 4 via a coupling lens 13, a polarized beam splitter 15, a λ/4 plate 16 and an objective lens 17. The reflected light travels backward to pass through the splitter 15 and is formed on into an image on a 4-split photodetector 26 via a light shielding plate 23 which is set at the position out of the optical axis and a lens 24. The shifting direction of a beam spot is detected via photodetecting cells 26aW26d in terms of the upper and lower directions and their orthogonal directions. In such way, the stable and simultaneous detection is possible for both the out-of-focus and a shifted tracking.