A method for 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 method inputting a first image and a second image from the two cameras, respectively, transforming the first image to a transformed 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, 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, detecting the obstacle based on the degree of similarity D and the degree of similarity P on a reference plane region, and determining a width and a height of the processing region on the basis of a position in the vertical direction on the second image.