An automatic playing system creates a feedback control loop for the keys incorporated in an acoustic piano; key sensors, which are provided under the front portions of the keys, informs a motion controller of current positions, and the motion controller periodically compares the current position and a current velocity with a target position on a reference trajectory and a target velocity to see whether or not a positional deviation and a velocity deviation occur; when the motion controller finds the deviations, the motion controller multiplies the deviations by a position gain and a velocity gain for determining an increment or decrement of the duty ratio of driving signals, and supplies the driving signals to the solenoid-operated actuators so as to accelerate or decelerate the keys; the gain is variable depending upon the key motion so that the actual key trajectory becomes close to the reference trajectory.