This paper describes an active camera system for high-speed object tracking. Under ordinary illumination, such as fluorescent lights, our system is capable of tracking an object in a typical indoor environment that contains a cluttered background. In order to accomplish this task, we developed a high-sensitivity high-frame-rate visual sensor system and a real-time object tracking algorithm that makes use of our previously proposed motion estimation technique, which is capable of robustly estimating optical flow in a high-frame-rate image sequence. The experimental results for our active camera system show the effectiveness and robustness of our visual sensor system and tracking algorithm.