Category determination is performed on an object candidate extracted from a captured image after an exclusion process is performed in advance. In the exclusion process, a distance from a vehicle to the object candidate is calculated based on a size of the object candidate in a current image, a moved distance of the vehicle occurring in a time interval from a time a previous image was captured to a time the current image is captured is calculated, the size of the object candidate in the current image is decreased at a change ratio based on the distance to the object candidate and the moved distance, and a size of the object candidate in the previous image and a size of the object candidate decreased at the change ratio are compared. The object candidate with difference between them being determined to be larger than a predetermined value is removed.