In order to define movement of a particular key to be automatically operated, first-order trajectory data are generated, on the basis of performance information, which are indicative of variation over time of position, velocity and acceleration components of the particular key. Jerk component related to the movement of the particular key is calculated on the basis of the acceleration component in the first-order trajectory data, and second-order trajectory data are generated by modifying the first-order trajectory data with the calculated jerk component. Then, a drive device for driving the particular key is servo-controlled on the basis of the second-order trajectory data. In this way, nonconstant acceleration (or constant jerk) control, rather than constant acceleration control, can be performed to control driving of the key during a successive key depression, so as to allow the successively-depressed key to operate with an increased smoothness.