1. A method for detecting a side of an object in an apparatus for detecting a side of an object within an image, comprising: analyzing, by a processor, an input image to search a detection target region in which a target object is included; detecting, by the processor, a boundary line between the target object and a ground within the detection target region; calculating, by the processor, a gradient for the boundary line; analyzing, by the processor, an image within the detection target region on the basis of the gradient to determine that the entirety of the detection target region is a region for the side of the target object or to determine a separated region corresponding to the side of the target object in the detection target region; and combining, by the processor, images of detection target regions determined to be the region for the side of the target object with each other so that the detection target regions are put on a continuous plane of the same object.