At the time of data recording onto a disk, when an external power source connection detecting circuit (12) detects that connection of an external power source is released, the currently set rotation speed is recognized. When the current rotation speed cannot be achieved in a maximum speed restricting mode, the processing speed is switched to the maximum speed that can be achieved in the maximum speed restricting mode. At the time of data reproduction on a disk, when the external power source connection detecting circuit (12) detects that the connecting power source is switched to the external power source, the processing speed is automatically switched to the maximum disk rotation speed regardless of the maximum speed restricting mode.