In order to precisely express motion of a key or motion of a hammer, a voice message for the polyphonic key pressure and another voice message for the control change, which stand idle in an automatic player piano, are used to express rough key position or rough hammer position and an offset from the rough key position or rough hammer position, and the offset is described at a high resolution on an ordinary trajectory between the rest position and the end position and at a low resolution outside of the ordinary trajectory; moreover, the numerical range expressed by the third byte of the voice message for the polyphonic key pressure is divided into two numerical sub-ranges respectively assigned to the keys and hammers so that only a few voice messages are required.