1. An apparatus for decoding motion information in merge mode, comprising: a merge mode motion information decoding unit configured to decode motion information using available spatial and temporal merge candidates when a motion information encoding mode of a current block indicates a merge mode; a prediction block generation unit configured to generate a prediction block of the current block using the decoded motion information; and a residual block decoding unit configured to generate a two-dimensional quantization block by inversely scanning residual signals, inversely quantize the two-dimensional quantization block using a quantization parameter, and generate a residual block by inversely transforming the inverse-quantized block, wherein a motion vector of the temporal merge candidate is selected among motion vectors of a first merge candidate block and a second merge candidate block based on a position of the current block within a slice or a largest coding unit, and the motion vector of the second merge candidate block is selected as the motion vector of the temporal merge candidate if the current block is adjacent to a lower boundary of the largest coding unit.