1. An apparatus for decoding motion information in merge mode, comprising: a merge mode motion information decoding unit configured to decode motion information when a motion information encoding mode of a current block indicates skip mode or 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 a residual signal, inversely quantize the two-dimensional quantization block using a quantization step size, and generate a residual block by inversely transforming the inverse-quantized block, wherein the quantization step size is reconstructed in a predetermined size unit.