1. An obstacle detection system comprising: an imaging unit for imaging the outside of a vehicle; and a processing unit for processing an image acquired from the imaging unit; wherein said processing unit executes the steps of: imaging a first image including a road surface image; imaging a second image including the road surface image at a timing different from the timing when the first image is imaged; creating top views of the first and second images; detecting a characteristic shape on the road surface from the two top views; aligning the two top views with each other on the basis of the detected characteristic shape; comparing overlapped portions of the two top views with each other; and judging an area in which a difference occurs to be an area including an obstacle.