A disk device, in which information is written by a magnetic head at an arbitrary position on a magnetic recording surface of a disk and the information written at the arbitrary position is read out, includes a control unit for controlling an operation of writing information at the arbitrary position on the magnetic recording surface of the disk and for controlling an operation of reading out the information written at the arbitrary position. The disk device also includes a modulation detecting unit for detecting that a modulation has been generated, by contact between the magnetic head and the disk, on the basis of a regenerative signal of a single frequency data region which is reproduced by a reproducing head element when the magnetic head is moved to a predetermined position in each track, wherein the single frequency data region is previously formed at the predetermined position in each track. On the other hand, a method of controlling the disk device, in which the above disk device is used, is disclosed.