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; 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, and wherein, 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.