PURPOSE: To attain stable tracking control without deteriorating the tracking gain by providing a sector detector and a changeover switch and allowing the sector to vary the gain of a variable gain amplifier.

CONSTITUTION: Signals from photodetectors 8, 9 enter a differential amplifier 10 to form a difference signal required for tracking and an objective lens 5 is displaced in the tracking direction by driving a tracking drive coil through a variable gain amplifier 11, a phase compensation circuit 12 and a drive amplifier 13 to attain tracking control. Then an output signal of an adder 14 enters a changeover switch 16 and a sector detector 15. When the signal enters the sector from a device except the sector, the sector detector 15 is active to throw the changeover switch 16 from a position (d) to a position (e) and the variable gain amplifier 11 has a fixed gain by a fixed voltage section 17. Thus, stable tracking control is applied similarly to the signal from the device other than the sector without deteriorating the tracking loop gain.