Performance interface system includes a motion detector provided for movement with a performer, and a control system for receiving detection data transmitted from the motion detector and controlling a performance of a tone in response to the received detection data. State of a performer's motion is detected via a sensor of the motion detector, and detection data representative of the detected motion state is transmitted to the control system. The control system receives the detection data from the motion detector, analyzes the performer's motion on the basis of the detection data, and then controls a tone performance in accordance with the analyzed data. With this arrangement, the performer can readily take part in the tone performance in the control system. For example, as the performer moves his or her hand, leg or trunk while listening to a manual or automatic performance of a music piece being carried out by a performance apparatus of the control system, the motion detector detects the performer's motion and transmits corresponding detection data to the control system, which in turn variably controls a predetermined one of tonal factors in the music piece performance. This arrangement can readily provide interactive performance control and thereby allows an inexperienced or unskilled performer to take part in a performance with enjoyment.