A three-dimensional object detection has an image capturing device, an image conversion unit, a three-dimensional object detection unit, a three-dimensional object assessment unit, first and second foreign matter detection units and a controller. The image capturing device captures images rearward of a vehicle. The three-dimensional object detection unit detects three-dimensional objects based on image information. The three-dimensional object assessment unit assesses whether or not a detected three-dimensional object is another vehicle. The foreign matter detection units detect whether or not foreign matter has adhered to a lens based on the change over time in luminance values for each predetermined pixel of the image capturing element and the change over time in the difference between an evaluation value and a reference value. The controller outputs control commands to the other means to suppress the assessment of foreign matter as another vehicle when foreign matter has been detected.