Disclosed is an apparatus and method for transmitting/receiving reference image data and 3D additional image data in 3D image data so as to provide a 3D image in a digital broadcasting system. In the method, a basic image and a 3D additional image in a 3D image of a 3D broadcasting service is processed in a stereoscopic image format of one of a side-by-side method and a top-down method. The basic image and the 3D additional image processed in the stereoscopic image format are encoded to a basic image stream and a 3D additional image stream. The encoded basic image stream and the encoded 3D additional image stream are multiplexed to a 3D image stream of a single stream using a dual stream method, and the multiplexed 3D image stream is then transmitted.