Keys, which are incorporated in an automatic player piano, are monitored with an optical transducer system. The optical transducer system includes sensor heads provided on both sides of the key trajectories, LEDs connected to predetermined sensor heads through optical fibers, LDDs connected to the other sensor heads through optical fibers and a controlling unit. A luminescence controller is connected to the LEDs for optimizing the luminescence, and bias controllers are respectively connected to the LDDs. The luminescence controller and bias controllers optimize the luminescence of emitted light and the bias level of electric signals so that the optical transducer system is free from the individuality of component parts and the deterioration.