A mute piano is responsive to key movements and pedal movements so as to produce music data codes expressing the pitch of tones and pedal effect to be given to the tones in accordance with the MIDI protocols, and the damper pedal stroke is divided into a rest region, a half-pedal region and a damper-free region which are expressed by difference numerical ranges of data bytes of the musical data code: since the piano has its own individuality, the relation between the pedal position and the value of data bytes is to be calibrated; the controller of the mute piano enters a calibration mode, and optimizes the relation to the actual damper actions, thereby causing the music data code exactly to express the effect of the damper pedal to be given to the tones.