An information recording/reproducing system has a data storage device that stores a data stream with a predetermined data format and an information recording/reproducing apparatus, which is connected with the data storage device, for recording and reproducing the data stream. The apparatus includes information recording/reproducing portion and memory portion that stores plural species of empty data file bodies each in which additional information corresponding to a species of data format and data are assigned to loading regions. The apparatus also has selection portion that selects an empty data file body with a specified data format from the plural species of empty data file bodies. The apparatus further has information-loading portion that loads the additional information and the data into predetermined loading regions assigned in the empty data file body that has been selected by the selection portion.