A parking frame drawn on a road surface is reliably detected. An imager (10) installed on a vehicle (1) captures an image (I) including at least a road surface around the vehicle (1). A candidate white-line area detector (30) detects, from the captured image (I), a candidate white-line area which is possibly a part of a parking frame. A parking-frame-similitude calculator (40) calculates the brightness distribution of the detected candidate white-line area and the surroundings of boundary points of a road surface. When the brightness distribution satisfies a certain condition, the parking-frame similitude or the degree to which the candidate white-line area forms a parking frame, is determined to be low. When a parking-frame similitude of the candidate white-line area forming a parking frame is detected in the image (I) by a parking-frame detector (60), the credibility (U) for the detected parking frame is set low.