An image decoding apparatus comprising: a parsing unit for decoding intra prediction information from a received bit stream; an intra prediction mode decoding unit 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 unit for creating a prediction block using the intra prediction mode, wherein the intra prediction information includes a prediction mode group indicator indicating whether the intra prediction mode of the current prediction unit belongs to the MPM group or not and a prediction mode index indicating the intra prediction mode of the current prediction unit among the three intra prediction modes of the MPM group when the prediction mode group indicator indicates the intra prediction mode of the current prediction unit belongs to the MPM group, and the decoding method of the prediction mode index varies depending on the prediction mode group indicator, and a size of the prediction block is determined according to transform information, the three intra prediction modes are determined according to intra prediction modes of left and top blocks of the current prediction unit, wherein when only one of the intra prediction modes of left and top blocks of the current prediction unit is present, two additional intra prediction modes are added to the MPM group; characterised in that the decoding apparatus is further configured to determine the three intra prediction modes as follows: when the intra prediction mode of the left block is not equal to the intra prediction mode of the top block, if the intra prediction modes of the left block and the top block are both directional intra prediction modes, the MPM group includes the intra prediction modes of the left block and the top block and a planar mode, and if one of the intra prediction modes of the left block and the top block is a DC mode and the other is the planar mode, the MPM group includes the DC mode, the planar mode and a vertical mode, and when none of the intra prediction modes of the left and top blocks is present, the MPM group consists of planar mode, DC mode and Vertical mode. a parsing unit for decoding intra prediction information from a received bit stream; an intra prediction mode decoding unit 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 unit for creating a prediction block using the intra prediction mode, wherein the intra prediction information includes a prediction mode group indicator indicating whether the intra prediction mode of the current prediction unit belongs to the MPM group or not and a prediction mode index indicating the intra prediction mode of the current prediction unit among the three intra prediction modes of the MPM group when the prediction mode group indicator indicates the intra prediction mode of the current prediction unit belongs to the MPM group, and the decoding method of the prediction mode index varies depending on the prediction mode group indicator, and a size of the prediction block is determined according to transform information, the three intra prediction modes are determined according to intra prediction modes of left and top blocks of the current prediction unit, wherein when only one of the intra prediction modes of left and top blocks of the current prediction unit is present, two additional intra prediction modes are added to the MPM group; characterised in that the decoding apparatus is further configured to determine the three intra prediction modes as follows: when the intra prediction mode of the left block is not equal to the intra prediction mode of the top block, if the intra prediction modes of the left block and the top block are both directional intra prediction modes, the MPM group includes the intra prediction modes of the left block and the top block and a planar mode, and if one of the intra prediction modes of the left block and the top block is a DC mode and the other is the planar mode, the MPM group includes the DC mode, the planar mode and a vertical mode, and when the intra prediction mode of the left block is not equal to the intra prediction mode of the top block, if the intra prediction modes of the left block and the top block are both directional intra prediction modes, the MPM group includes the intra prediction modes of the left block and the top block and a planar mode, and if one of the intra prediction modes of the left block and the top block is a DC mode and the other is the planar mode, the MPM group includes the DC mode, the planar mode and a vertical mode, and when none of the intra prediction modes of the left and top blocks is present, the MPM group consists of planar mode, DC mode and Vertical mode.