A computer readable storage medium storing instructions of a computer program which when executed by a computer results in performance of steps including inputting a first image and a second image from two cameras, respectively, transforming the first image to a transformed image, 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, 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 of the degree of similarity D and the degree of similarity P on a reference plane region, detecting the obstacle on a basis of the degree of similarity D and the degree of similarity P on a reference plane region, obtaining a difference K between the degree of similarity D and the degree of similarity P both obtained as being on the reference plane region, judging that the obstacle is at a position where the difference K is large, 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.