We developed a new system to capture jaw movement. It consists of two magnets and biaxial fluxgate sensors. A magnet is fixed to a tooth in the bottom jaw of the subject and another is fixed to the subject's forehead. The system can naturally detect movement of the jaw as a rigid body, because it does not require such attachments as a clutch inside the mouth or a magnetic field sensor array in contact with the head. In this paper, we investigate five error sources of the jaw-tracking system, using a jaw model, and report the achievement of a positional accuracy of 1.5 mm and an axial accuracy of 2 degrees. Positional accuracy within 2 mm is achieved as a rigid body.