1. An image decoding apparatus comprising: a parsing module for decoding intra prediction information from a received bit stream; an intra prediction mode decoding module for decoding an intra prediction mode of a current prediction unit using the intra prediction information and an MPM group including three intra prediction modes which are determined according to available left and above intra prediction modes; and a prediction block creating module for creating a prediction block using the intra prediction mode, wherein the intra prediction information includes a prediction mode group indicator and a prediction mode index, and the decoding method of the prediction mode index varies depending on the prediction mode group indicator, a size of the prediction block is determined according to transform information, and when intra prediction modes of left and top blocks of the current prediction unit are unavailable, the MPM group includes a planar mode, a DC mode and a vertical mode.