1. An apparatus for generating a prediction block, comprising: an entropy decoding unit for entropy-decoding a mode group indicator and a prediction mode index; an intra prediction mode decoding unit for 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; a reference pixel filtering unit for adaptively filtering reference pixels of a current block according to a size of a transform unit and the intra prediction mode; and a prediction block generating unit for generating the prediction block according to the intra prediction mode, wherein if the mode group indicator indicates the MPM group, the intra prediction mode decoding unit determines an intra prediction mode of the MPM group specified by the prediction mode index as the intra prediction mode of the current prediction unit, and if the mode group indicator does not indicate the MPM group, the intra prediction mode decoding unit derives the intra prediction mode of the current prediction unit by performing ordered 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 index 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.