In order to precisely detect latent defects where a data-synchronizing signal is to be stored despite changes in the rotation of an information storage medium, a region data detection is used during a defect detection mode that is longer in the back-and-forth direction than a data-synchronizing signal used during a normal operation mode. The data detection region may be increased by the amount of change in the rotation. Thus, latent defects that may, due to a change in the rotation of the storage medium, come into agreement with the position of a data-synchronizing signal utilized during a normal mode of operation may be reliably detected, maintaining a high precision by taking a change in the rotation of the storage medium into consideration.