Black/white keys are selectively moved by key actuators energized with a driving signal in a playback mode, and plunger sensors, which are provided inside of the key actuators, report the measured values of the keystroke to a motion controlling section; since the measured values contain error due to the deformation of the keys, the motion controlling section estimates true values or estimated values of the keystroke by dividing the sum of products between the measured values and weighting factors by a normalizing constant, and compares the estimated values with target values to see whether or not the keys exactly travel on reference trajectories; if the answer is negative, the motion controlling section varies the duty ratio of the driving signal so as to accelerate or decelerate the keys.