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 a mode group other than the MPM group and a prediction mode index indicating a specific intra prediction mode in a mode group indicated by the intra prediction mode group indicator among the MPM group and the mode group other than 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, when only one of the intra prediction modes of left and top blocks of the current prediction unit is available, two additional intra prediction modes are added to the MPM group, characterised in that the decoding apparatus is configured to determine the three intra prediction modes such that when one of the intra prediction modes of left and top blocks is a DC mode and the other is not a planar mode, the MPM group includes the intra prediction modes of left and top blocks and the planar mode, and when both of the intra prediction modes of the left and top blocks are not present, the MPM group consists of the planar mode, the DC mode and a 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 a mode group other than the MPM group and a prediction mode index indicating a specific intra prediction mode in a mode group indicated by the intra prediction mode group indicator among the MPM group and the mode group other than 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, when only one of the intra prediction modes of left and top blocks of the current prediction unit is available, two additional intra prediction modes are added to the MPM group, characterised in that the decoding apparatus is configured to determine the three intra prediction modes such that when one of the intra prediction modes of left and top blocks is a DC mode and the other is not a planar mode, the MPM group includes the intra prediction modes of left and top blocks and the planar mode, and when both of the intra prediction modes of the left and top blocks are not present, the MPM group consists of the planar mode, the DC mode and a vertical mode.