PURPOSE: To increase a positioning accuracy in a rough seek servo in which a carriage is shifted to the vicinity of the radial position of a target track address.

CONSTITUTION: In the optical disk device in which an optical pickup 3 to be shifted in the radial direction of an optical disk 2 is shifted to a prescribed position of the optical disk 2 by a seeking operation, a clock signal CK is formed from a positional signal P produced on the basis of an A-phase signal A and a B-phase signal B supplied from an encoder 4A detecting the position of the optical pickup 3 in the radial direction R and the positioning of the rough seek servo is performed by the unit of the clock signal CK. Thus, the positioning accuracy of this device is increased by four times as compared with a conventional technique in which the positing servo of the seeking operation is performed by the unit of a clock signal produced according to the A- phase signal and the B-phase signal.