Abstract: An eccentricity amplitude of a medium in an eccentricity measuring unit and an eccentricity phase for a start position of one rotation are measured. A region from the start position to end position of one rotation of the medium which was measured is divided into a plurality of regions every predetermined rotational angle, addresses are sequentially allocated to those regions, a sine value of a rotational angle which is increased every predetermined rotational angle has been stored in each address in an eccentricity memory. A read control unit forms an address (a) in an eccentricity memory corresponding to a rotating position of the medium where a light beam is irradiated at present while using the start position of one rotation of the medium as a reference and reads out a corresponding sine value sin&thgr;a from the eccentricity memory by the designation of the address (a).

Abstract: An optical storage apparatus has an eccentricity measuring unit that measures an eccentricity amplitude of a medium and an eccentricity phase for a start position along one rotation of a disk. A region from the start position to an end position of one rotation of the medium which was measured is divided into a plurality of regions every predetermined rotational angle, and addresses are sequentially allocated to those regions. A sine value of a rotational angle, which is increased every predetermined rotational angle, has been stored in each address in an eccentricity memory. A read control unit forms an address (a) in an eccentricity memory corresponding to a rotating position of the medium where a light beam is irradiated at present while using the start position of one rotation of the medium as a reference and reads out a corresponding sine value sin.theta.a from the eccentricity memory by the designation of the address (a).