In an automatic player piano, hammer sensors monitor associated hammers so as to report current positions on the hammer trajectories, and a data processor analyzes the hammer motion for producing pieces of music data representative of the performance on the acoustic piano; the aged deterioration is influential in the relative position between the hammers and the hammer sensors so that the data processor rectifies the relative position, the data processor determines the turning point at which the hammer changes the direction of motion, and compares the current value indicating the turning point with the previous value; if the difference is found, the data processor adds a value of deflection of strings to the current value so as to determine and memorizes the true value of the turning point; the data processor analyzes the hammer motion on the basis of the true value so that the music data are reliable.