An attachment part is disposed on a part of the edge of a resonance plate, which is made of a metal containing copper, and a weight is attached to the resonance plate at a predetermined distance from the attachment part. Plural vibration pickups are attached to the surface of the resonance plate between the weight and the attachment part. The resonance plate is attached to a percussion instrument at the attachment part. The plural vibration pickups produce electric signals corresponding to the resonant vibrations at their respective positions on the resonance plate, and the electric signals are mixed and output as percussion-instrument-sound signals.