a storage space provision portion, which provides, as a file, to the higher-level device, a single storage space associated with a plurality of logical storage units (LUs) provided by said storage system;

a reception portion, which receives from said higher-level device the file write requests to said provided storage space; and

a storage control portion, which selects an LU from among said plurality of LUs associated with the storage space specified by one of said file write requests, and stores in the selected LU all the data constituting said file according to said one of said file write requests,

wherein said storage control portion checks the file characteristics of each file stored in said plurality of LUs, identifies first LU characteristics matching the file characteristics of a checked file, and if the checked file exists in a second LU with second LU characteristics different from a first LU with said identified first LU characteristics, causes the checked file to be migrated from said second LU to said first LU,

wherein said storage system provides an automatic capacity-expansion LU to said file storage control device, and allocates unallocated storage areas among a plurality of storage areas to the automatic capacity-expansion LU, according to writing to the automatic capacity expansion LU, wherein said automatic capacity-expansion LU is included in said plurality of LUs, wherein said file storage control device further comprises a file size management portion which manages, for each file, an average increase size, which is a file size increase per unit time due to updating, and wherein said storage control portion causes files with an average increase size equal to or greater than a prescribed value to be migrated to said automatic capacity-expansion LU.