The invention provides a method for effectively removing the cause of a thermal asperity (TA) phenomenon occurring at an MR head incorporated in a disk storage system. If a read error occurs in data read by the MR head, a CPU executes a usual read retry operation. If the read error is not removed by the usual read retry operation, the CPU presumes that the read error is caused by the thermal asperity phenomenon occurring at the MR head. Then, the CPU controls the MR head to move to a CSS zone on the disk, thereby executing a TA removal operation so as to remove a fine particle such as dust attached to the MR head. In the TA removal operation, the rotational speed of the disk is reduced so that the MR head can be brought into contact with the disk in the CSS zone.