1. A roadside object detection apparatus to detect a roadside object using a plurality of captured images, comprising: a processor programmed to: analyze the captured images, and detect height information of an object in the captured images in a neighboring region, the neighboring region having a distance less than or equal to a threshold from a vehicle having the roadside object detection apparatus installed; detect a change of a height of a road surface due to the roadside object from the height information in the neighboring region; determine positions having the change of the height detected as characteristic points of the roadside object, define a road model based on the characteristic points, and extrapolate the road model into a far-off region, the far-off region having a distance greater than or equal to the threshold from the vehicle; set a search range for the roadside object in the far-off region based on a virtual line that is obtained by the extrapolated road model being, and detect the roadside object in the far-off region based on the search range; set the search range for the roadside object by translating the virtual line by predetermined distances to opposite directions in a symmetric or non-symmetric manner with respect to the virtual line; and detect the roadside object in the far-off region by applying template matching to the search range with a template image for the neighboring region, the template image being cut off from the captured images and including the roadside object in the neighboring region.