An image decoding method according to the present invention comprises: reconstructing an intra prediction mode group indicator and a prediction mode index of the current block; creating a first group with valid intra prediction modes of blocks left and above the current block; and, from the group indicated by the intra prediction mode group indicator, selecting a intra prediction mode corresponding to the prediction mode index as the intra prediction mode of the current block, wherein the size of a prediction block is in accordance with the information regarding the transformation size. Therefore, on the basis of the valid intra prediction modes of blocks left and above the current block, the intra prediction mode group indicator and the prediction mode index are determined by including, in the first group, the modes with high probabilities of being identical to the intra prediction mode of the current block, thereby reducing the amount of intra prediction information to be encoded.