1. An object detection system, comprising: a geometric information generation unit for calculating relationship information between a position on an image of an object existing on a road surface and an image area size of an image picked up by one camera, using an actual size of a target object and camera parameter information; a symmetry evaluation unit for evaluating an existence possibility of a symmetry axis in a specific pixel in case of an interval size determined on the basis of geometric information; a symmetry-axis continuity evaluation unit for evaluating density and continuity in the vicinity of the center axis of a rectangular area of a pixel whose symmetry value evaluated by the symmetry evaluation unit is equal to or more than a specific threshold and detecting the lowest end point of the symmetric object; and a rectangular area generation unit for outputting a position and size of a candidate area of the symmetric object on the basis of the lowest end point specified by the symmetry-axis continuity evaluation unit and the geometric information.