A method of image decoding can include generating a quantization block by inversely scanning quantization coefficient information; generating a transform block by inversely quantizing the quantization block using a quantization step size; generating a residual block by inversely transforming the transform block; reconstructing an intra prediction mode group indicator and a prediction mode index of a current block; constructing a first group including three intra prediction modes using valid intra prediction modes of left and top blocks of the current block; determining the intra prediction mode corresponding to the prediction mode index in the first group as the intra prediction mode of the current block when the intra prediction mode group indicator indicates the first group; generating a prediction block on the basis of the determined intra prediction mode of the current block; and generating a reconstructed block using the residual block and the prediction block.