1. An obstacle detecting system detecting an obstacle by using two cameras, the obstacle being on a reference plane in a field of view common to the two cameras, the system comprising: image inputting means for inputting a first image and a second image from the two cameras, respectively; image transforming means for transforming the first image to a transformed image on a basis of image transform which is introduced from a geometrical relationship between the 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; similarity calculating means for obtaining: a degree of similarity D indicating 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, on an assumption that 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 being established 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, and a degree of similarity P indicating 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, the processing region being established 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; and obstacle judging means for detecting the obstacle on a basis of the degree of similarity D and the degree of similarity P on a reference plane region obtained by the similarity calculating means.