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; and an approaching-object classification unit configured to classify the detection result of the object detection unit as an approaching object or not.