PURPOSE: To shorten a time required for the restoration of buffer memory by referring to a history table which manages the latest update time and the latest saving time in each area of the buffer memory, respectively, and saving data in an area decided to be saved.

CONSTITUTION: A saving decision means 13 decides whether or not the data in each area of the buffer memory 31 is saved to the corresponding area of a magnetic disk device 4 based on the content of the history table 22 at every issuance of a start-up instruction by a time monitoring means 12. A saving means 14 saves the data in the area decided to be saved from the buffer memory 31 to the corresponding area of the magnetic disk device 4. An invalid area decision means 17 refers to the history table 22 when a fault such as power failure, etc., occurs, and decides whether or not the data in each area of the buffer memory 31 saved to the magnetic disk device 4 is valid or invalid. In such a way, it is possible to shorten the time required for the restoration of the buffer memory.