When a storage capacity of a file server is expanded using an online storage service, elimination of an upper-limit constraint of the file size as a constraint of the online storage service and reduction in the communication cost are realized. A kernel module including logical volumes on the online storage service divides a file into block files at a fixed length and stores and manages the block files to prevent the upper-limit constraint of the file size. When a READ/WRITE request is generated for a mounted file system, only necessary block files are downloaded and used from the online storage service based on an offset value and size information to optimize the communication and realize the communication cost reduction.