An upright piano is equipped with a soft pedal, and a player makes the hammer stroke shorter by depressing the soft pedal; while a user is reproducing a music tune by means of an automatic player piano fabricated on the basis of the upright piano, the keys are servo controlled on the basis of a position difference of keys between target values and actual values and a key velocity difference, and the duty ratio of driving signal, which is supplied to solenoid-operated key actuators for the keys, are determined on the basis of multiplications between the position difference/key velocity difference and a position gain and a velocity gain; the value of position gain is reduced on the condition that the soft pedal is depressed for preventing the playback from an unintentional loud tone or tones.