PURPOSE: To easily and exactly position photodetectors relative to a light beam by positioning the parting line of the one photodetector perpendicularly to the parting line of the other photodetector.

CONSTITUTION: The I-component light receiving part 16 of the photodetector 15 is bisected to left and right light receiving parts 16b, 16c by the parting line 16a extending in a longitudinal direction so as to pass the center of a spot and the J-component light receiving part 17 is bisected to upper and lower light receiving parts 17b, 17c by the parting line 17a extending in a transverse direction so as to pass the center of the spot. The light beam from a light emitting and light receiving element 11 is reflected by a parallel flat plate type 2-beam Wollaston 12, is then polarized by a mirror 13, is refracted by an objective lens 14 and is focused at an optical disk 18. A part of the return light beam from this disk 18 is made incident on the element 11. The position where the difference (I1-I2) of the output signals of the light receiving part 16 is zero and (I1+I2) is not zero is detected by monitoring the difference (I1-I2), by which the positioning relative to the lateral direction of the photodetector 15 is executed.