1. An apparatus for decoding a video by using data unit merging, the apparatus comprising: an obtainer which obtains, from a received bitstream, a skip flag for a coding unit which indicates whether a prediction mode for the coding unit is a skip mode; and a determiner which determines motion information for a prediction unit, wherein: when the skip flag indicates a non-skip mode, the obtainer obtains a merge flag for the prediction unit in the coding unit, from a received bitstream, when the merge flag for the prediction unit indicates a merge mode, the obtainer obtains a merge index for the prediction unit from the bitstream, and the determiner determines motion information for the prediction unit using the merge index obtained from the bitstream, and when the merge flag for the prediction unit indicates a non-merge mode, the obtainer obtains motion information for the prediction unit from the bitstream, the merge index indicates one block among candidate blocks adjacent to borders of the prediction unit, in a merge candidate group and the candidate blocks are decoded prior to the prediction unit according to a decoding order, the merge candidate group includes a predetermined number of available candidate blocks on predetermined locations adjacent to the border of the prediction unit according to an implicitly predetermined order, and the image is split into a plurality of maximum coding units, according to information about a maximum size of a coding unit, the maximum coding unit is hierarchically split into one or more coding units of depth including at least one of a current depth and a lower depth, according to split information, when the split information indicates a split for the current depth, the coding unit of the current depth is split into four rectangular coding units of a lower depth, independently from neighboring coding units, and when the split information indicates a non-split of the current depth, one or more prediction units are obtained from one of the coding units of the current depths.