It is determined whether user's operation performed on a control is normal or special operation. A function of increasing or decreasing a value of a parameter assigned to the control is performed if the user's operation is the normal operation. Another function, different from the function of increasing or decreasing a value of a parameter, is performed if the user's operation is the special operation. The control comprises, for example, a touch pad provided with a display device for displaying a current operating position of the control. If a user touches one point of the pad with its finger or touches one point of the pad and then continuously slides its finger on and along the pad, such user's operation is determined to be the normal operation. If the user touches or taps two or more points of the pad, such user's operation is determined to be the special operation.