PURPOSE: To effectively utilize unused areas formed and dispersed on a magnetic disk by possessing and releasing the areas of a file.

CONSTITUTION: This device is provided with an unused area management part 1 to manage the unused areas dispersed on the magnetic disk device by possessing and releasing the areas of the file, used area management part 2 to manage the areas used at present, optimum reprogramming information calculation part 5 to calculate empty areas in a designated size while referring to the unused areas and the used areas, and reprogramming execution part 6 to execute reprogramming while receiving the output information of this optimum reprogramming information calculation part 5.