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