PURPOSE: To facilitate the positioning adjustment by arranging a split zone of a couple of photodetection faces being one pair of a four-split detector in a direction in parallel with a ridge line of a Foucault prism and arranging the split zone of the other pair of photodetection faces with a tilt to the ridge line of the Foucault prism.

CONSTITUTION: The beam from a laser light source 1 is collected to an information recording medium 6 by an objective lens 5, its reflected beam is reflected by a polarized beam splitter 2, divided into two by the Foucault prism. The split beams are given respectively to two photodetection faces 13b, 13c at the inner side of the four- split photodetector 13 and the two photodetector faces 13a, 13b at the outside of the detector 13, the summed signal is subject to differential amplification by a differential amplifier 11 to obtain a focus error signal 12 of zero/minus/plus polarity in response to the state of the objective lens 5 to the information recording medium 1 such as focusing point/near/remote state. The error of the interval between the two spots at the photodetector faces of photodetector 13 caused by the arrangement error of optical components is adjusted by displacing the photodetector faces 13c, 13c, provided with a tilt to the ridge line of the prism 7 in parallel with the ridge line of the prism 7.