An approach to reducing overhead to enable high-speed writing even when a large number of small-sized files are written onto a tape in a file system. The method of the present invention includes the steps of: writing multiple files consecutively onto a tape to become one combined file; writing, onto the tape, first index information including the start position and size of the combined file on the tape; and writing, onto the tape, second index information including the start position and size of each of the multiple files in the combined file on the tape.