An optical transducer includes a photo-electric converter supported by a shank flange rail and optical modulators respectively attached to hammers of an automatic player piano; since a neck portion makes a photo-modulating pattern closer to the rotational axis of each hammer than a boss portion adhered to the side surface of the hammer shank, the photo-modulating pattern is rotated in a narrow space in the vicinity of the rotational axis so as to permit the photo-electric converter to monitor the hammer motion between the rest position and the end position without any interference with other component parts, and the boss portion is easily adhered to the side surface under the condition that a worker lifts the hammer over the adjacent hammers.