1. A feature extraction system for extracting a feature from an image by computer processing, the system comprising: means for receiving an image and measured acceleration data from a mobile device including image acquiring means and acceleration measuring means; means for obtaining a gravity vector in the image in a camera coordinate system on the basis of the measured acceleration data; means for obtaining a vanishing point in the image in a vertical direction in a screen coordinate system by using the gravity vector; means for obtaining differential vectors along two axes for each of pixels in the screen coordinate system; means for obtaining a connection line vector connecting each of the pixels with the vanishing point; means for identifying a vertical edge on the basis of determination that an angle formed by the differential vector and the connection line vector is within a certain threshold range; means for obtaining the sum of strengths of vertical edges and writing the sum in a predetermined variable array; means for extracting a keypoint on the basis of the variable array; and means for calculating a feature quantity from the keypoint.