In this paper, we present a novel approach to estimate the human's eye gaze from a single infrared spectrum image of two eyes. Observing that the pupil contour is a circle, we estimate the normal direction of this circle, considered as the eye gaze, from its elliptical image. By assuming that the two gaze vectors of both eyes and the vector which is determined by the two centers of the right and left pupil are coplanar, we can calculate the both eye gaze. Compared to most existing 3-D gaze estimation algorithms, our approach does not require those of face feature information such as the eye corners. Since the focal length can be calculated by our "Coplanar Constraint", ours does not need to know the exact focal length of the camera. The extensive experiments over simulated images and real images demonstrate the feasibility and the effectiveness of our method.