An arbitrary signal insertion method and an arbitrary signal insertion system, capable of inserting a transmittable arbitrary signal (insertion information M) at a predetermined insertion timing into an acoustic sound played in real time. An insertion timing is previously associated with a predetermined time code with master rhythm information. An acoustic sound into which insertion information will be inserted is music sound generated by a real-time performance unit and is accompanied by a second rhythm. The insertion information is inserted into the music sound generated by the real-time performance unit at the insertion timing after the rhythm of master rhythm information and the rhythm of the music sound generated by the real-time performance unit are synchronized. The synchronization is achieved by a rhythm transmitter which notifies a player of a rhythm session musical instrument of the rhythm of the master rhythm information with sound or light.