That is to say, the probability of the layer jump is decreased by rapid disk deflection.Next, the operation of the rotation control circuit 51c according to the second embodiment of the present invention is explained using FIG. 4, FIGS. 9A 9E, FIG. 17, and FIGS. 18A 18E. However, description of the operation which is similar to thefirst embodiment of the present invention is omitted.(A) To begin w