To precisely detect a tracking error signal irrespective of the track spacing of an optical disk.

In a photodetector 9, in generating a tracking error signal, a light beam from a semiconductor laser is divided into a zeroth-order diffracted light beam, a +first-order diffracted light beam and a -first-order diffracted light beam by a diffraction grating and reflected from an optical disk, and then the zeroth-order diffracted light beam is detected in a light receiving area 15 which consists of light receiving planes 15ab and 15cd placed a spacing a0 apart, the +first-order diffracted light beam is detected in a light receiving area 16 which consists of light receiving planes 16a and 16b placed a spacing b0 apart, and the -first-order diffracted light beam is detected in a light receiving area 17 which consists of light receiving planes 17a and 17b placed a spacing c0 apart. Arithmetic operations are carried out to these detected signals respectively, and a tracking error signal is generated using a push-pull method.