A file system corresponding to virtual hard disk files is identified, and a file size of the virtual hard disk files after optimization is calculated from the size of the volume in use of the file system so that a volume size can be ensured by the optimization is calculated. Specifically, a management server performs the above-described processing, and has the following structure. The management server is connected to a physical server that is virtually composed of virtual servers including virtual hard disks. The management server comprises a volume acquiring unit that acquires the volume in use of a first file system on the virtual hard disks; a file size acquiring unit that acquires the file size of virtual hard disk files corresponding to the first file system; and a calculating unit that calculates an optimized file size of the virtual hard disk files on the basis of a difference between the acquired volume in use and the file size.