1. A method for detecting an obstacle by using two cameras of three or more cameras, the obstacle being on a reference plane in a field of view common to the two cameras, the method comprising: inputting a first image and a second image from the two cameras, respectively; transforming the first image to a transformed image on a basis of image transform which is introduced from a geometrical relationship between a reference plane and the two cameras for transforming any given pixel in a reference plane region in the first image to a corresponding pixel in the second image; obtaining a degree of similarity D indicating a similarity between an image in a processing region established in the second image and an image in a corresponding processing region established in the first image and being a function of position in the vertical direction of the image; establishing, when any given image line in a specified region in the second image is a contact line of the obstacle with the reference plane, the processing region for an inter-image arithmetic operation between the first image and the second image, and a plurality of the processing regions being established with the image line being shifted in the vertical direction; obtaining a degree of similarity P indicating a similarity between an image in the processing region established in the second image and an image in a corresponding processing region established in the transformed image and being a function of position in the vertical direction of the image; establishing the processing region for an inter-image arithmetic operation between the second image and the transformed image, and a plurality of the processing regions being established with the image line being shifted in the vertical direction; detecting the obstacle on a basis of the degree of similarity D and the degree of similarity P on a reference plane region; and selecting and outputting either a result of detection of a plurality of the obstacles or a position of an obstacle detected as being the closest to the three or more cameras.