A storage system for optimizing volume capacity among file management computers of sharing a storage apparatus is provided. A capacity management computer acquires statistical information such as access history information from the file management computers and the storage apparatus. The capacity management computer executes a capacity optimizing process for the storage apparatus in association with a specified date. The capacity management computer calculates optimized capacity for the respective file systems based on the statistical information by redistributing available capacity of volumes according to a predetermined standard such as a capacity consuming speed. Further, the capacity management computer instructs the file management computers and the storage apparatus to optimize a size of the file systems and the capacity of the volumes, respectively.