1. An obstacle detection device in which at least two image pick-up devices each pick up an image of a preset common detection region, and which determines whether an obstacle is present or not in the detection region from the stereo images picked up using the image pick-up devices, the device comprising: image input means for receiving the image from each of the image pick-up devices; correspondence detection means for deriving a plurality of pairs of corresponding points in the detection regions of the received two stereo images; slope degree calculation means for calculating a slope degree corresponding to a slope angle between a basic plane which is parallel to optical axes of the image pick-up devices and a detection plane including the derived plurality of corresponding points in the stereo images; and result determination means for determining as there being an obstacle when the slope degree of the detection plane is larger than a predetermined value.