When the meta data in an MP4 file is converted, a media data recopying process involves a long process duration and large memory space allocation. The invention provides that when an MP4 file is converted, an increment of a converted file size is estimated based on previously-registered meta data conversion candidate information, an overwritable free space equivalent to the estimated incremental size is inserted before a media data portion. In the MP4 file conversion, the converted meta data overwrites the to-be-converted meta data portion and the overwritable free space such that the MP4 file is converted without moving the media data.