An apparatus and a method synthesize additional information while rendering an object in a 3D-based terminal. A disparity of the additional information is determined with a value that is equal to a disparity of an object or greater than the disparity of the object by a predetermined value in a situation in which the object hidden by the additional information is to be synthesized using a depth buffer. The additional information is overlaid on an image with consideration of the determined disparity of the additional information.