According to one embodiment, a disk controller includes a receiver, a read-modify-write controller, and a wait controller. The receiver receives data in units of predetermined data length. The read-modify-write controller performs a read-modify-write operation when the receiver receives the data to write the data to a sector on a disk medium the rotation of which is controlled. The sector is longer than the predetermined data length. The wait controller delays the start of the read-modify-write operation by the read-modify-write controller for within the time which the disk medium takes to rotate once and which is substantially the same as the time required to control the read-modify-write operation.