1. A method of embedding additional data in performance data to be used for reproduction of a music piece and storing the performance data and additional data in a readable form, said method comprising: a supply step of supplying performance data, said performance data comprising a plurality of kinds of event data, each of the event data representing any one of a plurality of kinds of processing instructions; an extraction step of extracting, from among the performance data supplied by said supply step, one or more particular kinds of event data; and a change step of sequentially changing the particular kinds of event data extracted by said extraction step, in accordance with a predetermined algorithm and using additional data to thereby embed the additional data into the performance data.