To change over an automatic play pattern without spoiling performance of percussion instrument play by a player by beating a drum pad so as to become a beat pattern conformed to a trigger pattern corresponding to every drum pad to change over the automatic play pattern.

When whichever drum pad within a plurality of drum pads 22-28 is beated, the beat pattern of the drum pad is compared with a trigger pattern stored in a trigger pattern storage means (RAM 16) and corresponding to the beated drum pad. When the result of comparison shows coincidence of the beat pattern with the trigger pattern, an automatic play pattern during automatically playing at present is changed over to whichever automatic play pattern stored in an automatic play pattern storage means (ROM 14). The automatic play is performed following to the changed-over automatic play pattern.