1. A contact time calculation apparatus, comprising: an image-capturing device installed at a mobile body; an evaluation point extraction device that extracts as evaluation points two arbitrary points belonging to a single object on an image captured by the image-capturing device; a first calculation device that calculates an absolute value representing a difference between coordinate values of the two points having been extracted from the image by the evaluation point extraction device, which are taken in reference to an arbitrary coordinate axis set on the image; a second calculation device that calculates a time differential value between at least two absolute values that are calculated from images captured at different times by the image-capturing device respectively, each of the absolute values representing the difference between the coordinate values of the two points calculated by the first calculation device; and a contact time calculation device that calculates a contact time being a length of time to elapse before the object containing the two points extracted by the evaluation point extraction device contacts the mobile body based upon the absolute value representing the difference between the coordinate values of the two points calculated by the first calculation device and the time differential value of the absolute value representing the difference between the coordinate values of the two points, which is calculated by the second calculation device.