Teaching images are acquired at a plurality of separate teaching points on a running route extending from a running start position to a goal position, respectively, under a first light environmental condition and a light environmental condition different from the first light environmental condition, and the teaching images are stored. A present teaching image serving as a target for a robot body in a running direction at present is selected from the stored teaching images. A driving mechanism is controlled so as to increase the matching degree between the present teaching image and an actual image taken by a camera.