A file server apparatus that is adapted to acquire a file from an online storage service connected via a network (102, 104) to provide the file to a user terminal, the file server apparatus comprising: a local cache that is adapted to store K (positive integer, K<N) divided files among N (positive integer) divided files generated by dividing an original file stored by the online storage service; and a mount module that is adapted to acquire data from the online storage service and the local cache, wherein the mount module is adapted to determine whether all divided blocks necessary to provide data corresponding to an I/O request exist in the local cache and to acquire the data corresponding to the file I/O request from the divided blocks if the necessary divided blocks exist; characterized in that: the local cache is further adapted to store metadata of the original file, the metadata including file update information indicating whether the original file is updated and bit map information indicating a state of the N divided files, and if the I/O request is a write request and at least one of the K divided files stored in the local cache is updated, the mount module is adapted to set both the file update information and the bit map information corresponding to the updated divided file to information indicative of the update; wherein the metadata further includes metadata update information indicating whether the metadata is updated and file size information indicating a file size, and if the I/O request is a file size change request, the mount module is adapted to determine whether the file size change request is an expansion request of the target file, is adapted to apply an expansion process or a reduction process to the bit map information based on the determination result, is adapted to update the file size information, and is adapted to set the metadata update information to information indicative of the update. a local cache that is adapted to store K (positive integer, K<N) divided files among N (positive integer) divided files generated by dividing an original file stored by the online storage service; and a mount module that is adapted to acquire data from the online storage service and the local cache, wherein the mount module is adapted to determine whether all divided blocks necessary to provide data corresponding to an I/O request exist in the local cache and to acquire the data corresponding to the file I/O request from the divided blocks if the necessary divided blocks exist; characterized in that: the local cache is further adapted to store metadata of the original file, the metadata including file update information indicating whether the original file is updated and bit map information indicating a state of the N divided files, and if the I/O request is a write request and at least one of the K divided files stored in the local cache is updated, the mount module is adapted to set both the file update information and the bit map information corresponding to the updated divided file to information indicative of the update; the local cache is further adapted to store metadata of the original file, the metadata including file update information indicating whether the original file is updated and bit map information indicating a state of the N divided files, and if the I/O request is a write request and at least one of the K divided files stored in the local cache is updated, the mount module is adapted to set both the file update information and the bit map information corresponding to the updated divided file to information indicative of the update; wherein the metadata further includes metadata update information indicating whether the metadata is updated and file size information indicating a file size, and if the I/O request is a file size change request, the mount module is adapted to determine whether the file size change request is an expansion request of the target file, is adapted to apply an expansion process or a reduction process to the bit map information based on the determination result, is adapted to update the file size information, and is adapted to set the metadata update information to information indicative of the update.