In a storage management system for managing a storage device for storing data, management information for managing the storage device, area assignment information for managing an area assignment state of the storage device, and history information for managing a history on area assuring and release are held. Further, an assignment request for temporarily using an area is received in association with designation of an area size and a using period. It is determined whether or not the requested area can be assigned by referring to information calculated from the held history information. An area with a designated size at a designated period is assigned to a request source if the area can be assigned as a result of the determination. In the determination, transition of a used amount of a storage is obtained from the history information, and it is determined based on the transition of the used amount of the storage whether or not the designated area can be assigned at the designated period.