PURPOSE: To attain accurate objective lens drive control by providing a prescribed balancer so as to make the driving point and its gravity center position acting on a drive section coincident with each other and providing a correction member correcting the deviation between the drive point and the gravity center position after balancer mounting.

CONSTITUTION: The balancer 17 having a prescribed weight and shape decided by design is fitted to a support member 3 to make the drive point and the gravity center position coincident with each other and the deviation between the drive point and the gravity center position based on the manufacture error after the mounting of the balancer 17 is corrected by a ring balancer 18. Further, the phase of the rolling resonance after the mounting of the balancer 17 is brought into the lead side, the ring balancer 18 having a thickness (t) in response to the resonance quantity of rolling is fitted to a flange 2a of a barrel 2 at the upper side of the moving gravity center to shift the phase toward the delay side thereby preventing the rolling resonance. Thus, the accuracy of the drive control of the objective lens 1 is improved.