According to one embodiment, a disk medium rotates in a first rotation state when access is given, and rotates in a second rotation state lower in rotation number than the first rotation state when access is not given. A controller receives, from a host device, a read command for reading first data stored in a first buffer, when the disk medium is in the second rotation state. The controller transmits the first data from the first buffer to the host device, without causing a shift into the first rotation state, and then shifts the disk medium into the first rotation state after completion of execution of the read command.