The memory area managing unit 22 (a) sets a protect flag to each virtual area allocated in a virtual memory space, the protect flag indicating whether a use of the virtual area has been finished or not, and (b) when a part or all of a first virtual area would overlap another second virtual area due to expansion or movement of the first virtual area, allows the expansion or the movement of the first virtual area accompanying with overlapping the second virtual area, if the protect flag of the second virtual area indicates that a use of the second virtual area has been finished. If the expansion or the movement is allowed, the memory pool managing unit 23 adds a physical area in a physical memory space corresponding to an overlapping part of the first and second virtual areas into a memory pool to map to another virtual area.