1. A method of decoding a stereoscopic image data stream including a main stream and an auxiliary stream, the method comprising: extracting composition time stamps of monoscopic image data regions from individual streams of the main stream and the auxiliary stream; if the monoscopic image data regions have the same composition time stamps, determining which stream of the individual streams is the main stream containing base view images; and reproducing monoscopic image data extracted from at least on monoscopic image data region the determined main stream, wherein each of the main stream and the auxiliary stream comprises at least one stereoscopic image data region and at least one monoscopic image data region.