1. A method of generating a prediction block, the method comprising: entropy-decoding a mode group indicator and a prediction mode index; constructing a Most Probable Mode (MPM) group including three intra prediction modes using a left intra prediction mode and an above intra prediction mode of a current prediction unit, and deriving an intra prediction mode of the current prediction unit using the mode group indicator and the prediction mode index; adaptively filtering reference pixels of a current block according to a size of a transform unit and the intra prediction mode of the current prediction unit; and generating the prediction block according to the intra prediction mode of the current prediction unit, wherein if the mode group indicator indicates the MPM group, an intra prediction mode of the MPM group specified by the prediction mode index is determined as the intra prediction mode of the current prediction unit, wherein if one of the left intra prediction mode and the above intra prediction mode is available, and the available intra prediction mode is a planar mode, the MPM group includes a DC mode, and wherein if the mode group indicator does not indicate the MPM group, the intra prediction mode of the current prediction unit is derived by performing ordered sub-steps of: increasing a value of the prediction mode index by one if the prediction mode index is equal to or greater than a first candidate of the MPM group; increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a second candidate of the MPM group; increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a third candidate of the MPM group; and determining the value of the prediction mode index as the mode number of the intra prediction mode of the current prediction unit.