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In this paper, we propose a high-performance, ultrasonic sensor-based head movement detection system, which can be easily applied as an eye tracking device by rotating a mirror set in front of a video camera in a head-free video-based eye-gaze detection system. We propose a simple distance measurement method that uses an A-D converter and an envelope detection method. Experimental results indicate small standard deviations of 0.4-0.8 mm (0.10% S.D.) when the distance between the transmitter and the receiver is within the range of 40-80 cm. In addition, we arranged three transmitters on the apices of a triangle to simulate the head of the user. The receivers were arranged on the apices of a larger triangle. The coordinates of each transmitter were determined using the measured distances from the transmitter to the three receivers. The center of gravity of the three transmitters and the normal vector of the plane that includes the three transmitters were calculated based on the estimated coordinates of the three transmitters. The results indicate sufficiently high precision and measuring frequency for our purpose. The proposed method would be useful for adjusting the focus of a zoom lens to the eye, as well as adjusting the direction of the mirror to the eye.