To appropriately execute the removal of the same phase in a wobble component of each double spiral track without bringing the request for enhancing the precision of an optical system.

After different gains are given to each of a detection signal (I) of a 1st reflected light and a detection signal (J) of a 2nd reflected light with respect to a disk, on which a 1st track and 2nd track are double-spirally formed and one side of each track is physically wobbled, a data signal corresponding to a magnetic field pit train is obtained by the subtracting process between the detection signals of the 1st reflected light and the 2nd reflected light. Especially, the gain ratio is changed over according to the reproducing time of the 1st track and the reproducing time of the 2nd track.