1. An apparatus for processing a depth image, the apparatus comprising: a processor configured to: obtain a depth image; extract an object image from the depth image; calculate a relative angle between an image sensor used to photograph the depth image and a target object corresponding to the object image; rotate the object image based on the relative angle and a reference angle, the reference angle being related to reference images prepared in advance to recognize the object image; and fill empty spaces comprised in the rotated object image based on at least one of an interpolation method and a filtering method, wherein the relative angle is determined based on a normal vector of a floor included in the depth image.