An automatic player piano is broken down into an acoustic piano and an automatic playing system, and solenoid-operated key actuators are driven to rotate the black/white keys of the acoustic piano; the space between the keyboard and the array of black/white keys is so narrow that the manufacturer provides the solenoid-operated key actuators in the space below the key bed; plunger holes are formed in the rear portion of the key bed, and the solenoids occupy the space under the key bed so that the plungers project through the plunger holes into the space beneath the rear portions of the keys; the key bed is does not lose the mechanical strength so that the tuning work is not frequently required for the key action units.