Disclosed is a video decoding apparatus including a merge mode motion information decoding unit that decodes motion information if a motion information encoding mode of a current block is a merge mode, a prediction block generation unit that generates a prediction block of the current block using the decoded motion information, and a residual block decoding unit that generates a quantization block by inversely scanning a residual signal to dequantize the quantization block using a quantization step size and to generate a residual block by inversely transforming the dequantization block, wherein the quantization step size is generated by using the average value of two effective quantization step sizes in the left side quantization step size, the upper side quantization step size, and the previous quantization step size, and if only one quantization step size of the left side quantization step size, the upper side quantization step size, and the previous quantization step size is effective, the quantization step size is decoded by using the effective quantization step size.