1. An approaching-object detection system for detecting an approaching object based on images captured by an imaging device, the approaching-object detection system comprising: an extraction unit configured to extract a partial image of one far side and a partial image of the other far side from each of the images; a distortion correction unit configured to correct distortion of the partial images extracted by the extraction unit; an object detection unit configured to detect an object from the partial images corrected in distortion by the distortion correction unit through pattern matching with reference to preinstalled image information on the object; an approaching-object classification unit configured to classify the detection result of the object detection unit as an approaching object or not; and an orientation estimation unit configured to estimate an orientation of the imaging device, wherein the approaching-object classification unit is configured to: calculate a feature value of the object detected by the object detection unit; and classify the object as approaching object or not based on a variation with time in the calculated feature value of the object, and wherein the approaching-object detection unit is configured to: locate a grounding point of the object detected by the object detection unit in the partial images corrected in distortion by the distortion correction unit; calculate a relative distance from the imaging device to the object based on the orientation of the imaging device estimated by the orientation estimation unit and the located grounding point; estimate a pixel count of the object based on the calculated relative distance and a kind of the object detected by the object detection unit; and determine whether the object is an approaching object or not in consideration of a difference between the estimated pixel count and an actual pixel count of the object.