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 a spatial merge candidate 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 a merge candidate 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 list of merge candidates using the spatial merge candidate derived by the merge candidate derivation unit, the temporal merge candidate generated by the temporal merge candidate configuration unit, and the merge candidate generated by the merge candidate generation unit and select a merge predictor based on the merge predictor index reconstructed by the merge predictor index decoding unit; and a motion information generation unit configured to generate a reference picture index and a motion vector of the current block from those of a merge candidate, the merge candidate being selected by the merge predictor selection unit.