The inverted-stereo effect is a type of visual fatigue caused by reversed stereoscopy - i.e., the left-view image is delivered to the right eye and the right-view image to the left eye. Because the human visual system has no mechanism for recognizing inverted-stereo, the human brain struggles to overcome the oddness but ends up feeling strong visual discomfort. We propose a feature-based method for estimating the relative positions of stereo images. The proposed method is based on the properties of epipolar geometry and finds the relative positions from a pair of stereo images without prior knowledge of camera configurations. Furthermore, its computational complexity is relatively low because most of the required computation can be conducted by well-known linear algebraic operations.