This paper describes new vision-based registration methods utilizing not only cameras on a user's head-mounted display but also a bird's-eye view camera that observes the user from an objective viewpoint. Two new methods, the Line Constraint Method (LCM) and Global Error Minimization Method (GEM), are proposed. The former method reduces the number of unknown parameters concerning the user's viewpoint by restricting it to be on the line of sight from the bird's-eye view. The other method minimizes the sum of errors, which is the sum of the distance between the fiducials on the view and the calculated positions of them based on the current viewing parameters, for both the user's view and the bird's-eye view. The methods proposed here reduce the number of points that should be observed from the user's viewpoint for registration, thus improving the stability. In addition to theoretical discussions, this paper demonstrates the effectiveness of our methods by experiments in comparison with methods that use only a user's view camera or a bird's-eye view camera.