An automatic player piano is a combination between an acoustic piano and an electronic system serving as a recorder and an automatic player; while a user is fingering a piece of music on the acoustic piano, the player strongly depresses some keys and softly depresses other keys so as to give artistic expression to the tones, and the depressed keys give rise to hammer motion at different values of hammer velocity; since the hammer velocity is proportional to the loudness of tones, the controller is expected exactly to determine the hammer velocity; the hammers travel on linear trajectories and parabolic trajectories depending upon the hammer velocity so that the controller estimates the hammer velocity by using different methods, whereby the controller can produce music data codes exactly expressing the tones to be produced.