To store main information with associated additional information incorporated therein, data constituting the additional information is divided into a plurality of small-size data pieces of, e.g., one bit. Then, the respective values of particular ones of predetermined data units (e.g., bytes) constituting the main information are subjected to arithmetic operations in accordance with a predetermined algorithm containing the value of each of the data pieces as a parameter. In this way, the respective values of the particular data units in the main information are modulated in accordance with the values of the individual data pieces in the additional information; at that time, only some of the data unit values are altered with the others left unaltered. The thus-arithmetically-operated main information is stored into a storage. For reproduction of the additional information, the stored main information is read out, and the individual data pieces are reproduced from the particular data units by inversely executing the algorithm. For example, the main information includes MIDI key-on event data, program change data or control change data, waveform data, or image data, and the additional information includes letter data representative of the name of an author of a music piece in question, title of the music piece or title of an image and data representative of a data format employed.