1. An on-vehicle image processor comprising: an imager mounted on a vehicle, which observes surroundings of the vehicle and captures an image including at least a road surface; a candidate white-line area detector which detects positions of a first pixel and a second pixel while scanning the image captured by the imager in a certain direction, the first pixel of which a brightness value changes to be brighter and exceeds a predetermined value, the second pixel being located within a certain distance from the first pixel, of which a brightness value changes to be darker and exceeds a predetermined value, and detects, as a candidate white-line area forming a parking frame, an area surrounded by both end points of a linear element of the first pixels and both end points of a linear element of the second pixels; a parking-frame similitude calculator which calculates a parking-frame similitude according to a brightness distribution of surroundings of the first pixels of the candidate white-line area and a brightness distribution of surroundings of the second pixels, the parking-frame similitude being a similitude when each of the candidate white-line areas is assumed to form a parking frame; and a parking frame detector which detects a parking frame according to the position of the candidate white-line area, the parking-frame similitude, and an expected size of a parking frame in the image, wherein the parking-frame similitude calculator decreases the parking-frame similitude of the candidate white-line area when the brightness distributions of the surroundings of the first pixels and the second pixels of the candidate white-line area satisfy a certain condition.