PURPOSE: To accurately adjust the relative position of plural beams by arranging a total reflection member between a polarizing beam splitter (PBS) and an objective lens and arranging an image rotating prism between this member and the PBS.

CONSTITUTION: Three light beam emitted from an array type semiconductor laser 1 pass a collimator lens 2, and p-polarized light transmitted through a PBS 3 is transmitted through an image rotating prism 4 and is reflected at right angles by a total reflection prism 5 and is converged by an objective lens 6 and is made incident on a magneto-optical disk 7. Turning of the prism 4 is so controlled that three light beam are on the same track Tn of the disk 7. The reflected light from the disk 7 goes backward and is reflected by the BPS 3 and passes a condenser lens 8, a polarizing hologram 9, and a birefringence plate 10 and is made incident of a photodetector 11. The first-order diffracted light of the incident light is made incident of three light receiving elements 12-1 to 12-3 out of nine divided light receiving elements 12-1 to 12-9, and a tracking error signal is obtained where offset of p-polarized light and s-polarized light is cancelled.