This paper describes a new head tracking method utilizing a gyroscope mounted on a head-mounted display (HMD) and a bird's-eye view camera that observes the HMD from a fixed third-person viewpoint. Furthermore, we propose an extension of this method to hybrid registration, combining it with user's view cameras. The HMD is equipped with a gyroscope and a marker. The gyroscope measures the orientation of the user's view camera so that the number of pose parameters to be solved can be reduced. The other parameters are to be estimated by the bird's-eye view camera that observes the marker. This method is an improvement over the conventional outside-in-style vision-based tracker method, which only uses visual information. Hence, it can be thought of as an alternative to a physical head tracker such as a magnetic sensor and to an inside-out-style vision-based tracker. In addition to theoretical discussions, this paper demonstrates the effectiveness of our methods by experiments. We also propose methods for calibrating the gyroscope and the marker on HMD, which are essential in implementing the tracking method.