1. An apparatus for decoding motion information in merge mode, comprising: a merge predictor index decoding unit configured to reconstruct a merge predictor index of a current block using a received merge codeword; a spatial merge candidate derivation unit configured to derive spatial merge candidates of the current block; a temporal merge candidate configuration unit configured to generate a temporal merge candidate of the current block; a merge candidate generation unit configured to generate one or more merge candidates when the number of valid merge candidates of the current block is smaller than a predetermined number; a merge predictor selection unit configured to generate a merge candidate list using the merge candidates and select a merge predictor based on the merge predictor index; a prediction block generating unit configured to generate a prediction block of the current block using motion information of the merge predictor; and a residual block generating unit configured to perform an entropy-decoding process and an inverse-scanning process on residual signals to generate a quantized block, and to perform an inverse-quantizing process and an inverse-transforming process on the quantized block to generate a residual block, wherein the temporal merge candidate configuration unit is configured to set a reference picture index of the temporal merge candidate as 0, and 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.