A music data modifier receives pieces of original music data from a master hybrid piano, and partially modifies the pieces of original music data to pieces of modified music data for a slave hybrid piano; each piece of original music data contains a series of values of a piece of motion data expressing continuous motion of the associated key, a series of values of a piece of time data expressing each expressing a time to obtain the associated value of the motion data and a piece of identification code expressing the key number assigned to the key; even if the music data modifier changes the piece of identification data from the key number to another key number, the piece of motion data still expresses the continuous motion of the key so that the slave hybrid piano exactly reproduce the key motion.