A method of deriving an intra prediction mode of a current prediction unit, the method comprising: entropy decoding a mode group indicator and a prediction mode index, wherein the mode group indicator is a flag indicating whether the intra prediction mode of the current prediction unit belongs to a most probable mode, MPM, group and wherein the prediction mode index specifies the intra prediction mode of the current prediction unit within the group specified by the mode group indicator; constructing the MPM group using intra prediction modes of a left block and an above block of the current prediction unit, wherein the MPM group includes three intra prediction modes; determining whether the mode group indicator indicates the MPM group; determining an intra prediction mode specified by the prediction mode index among the three intra prediction modes in the MPM group as the intra prediction mode of the current prediction unit when the mode group indicator indicates the MPM group; and deriving the intra prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes in the MPM group when the mode group indicator does not indicate the MPM group, wherein if the prediction mode Index is greater than or equal to a first mode in the MPM group, the value of the prediction mode index is increased by one; if the prediction mode index is greater than or equal to a second mode in the MPM group, the value of the prediction mode index is increased by one; and then if the prediction mode index is greater than or equal to a third mode in the MPM group, the value of the prediction mode index is increased by one, wherein the MPM group includes two non-directional modes and vertical mode when only one of the intra prediction modes of the left and the above blocks is available and the available intra prediction mode is one of the two non-directional modes, wherein the two non-directional modes are planar mode and DC mode, wherein the first mode is an intra prediction mode of a lowest mode number in the MPM group, the second mode is an intra prediction mode of a middle mode number in the MPM group, and the third mode is an intra prediction mode of a highest mode number in the MPM group, wherein the three intra prediction modes in the MPM group are sorted in an order of the first mode, the second mode and the third mode; and wherein, when only one of the intra prediction modes of the left and the above blocks is available and the available intra prediction mode is one of directional modes, the MPM group includes the two non-directional modes and the available intra prediction mode. entropy decoding a mode group indicator and a prediction mode index, wherein the mode group indicator is a flag indicating whether the intra prediction mode of the current prediction unit belongs to a most probable mode, MPM, group and wherein the prediction mode index specifies the intra prediction mode of the current prediction unit within the group specified by the mode group indicator; constructing the MPM group using intra prediction modes of a left block and an above block of the current prediction unit, wherein the MPM group includes three intra prediction modes; determining whether the mode group indicator indicates the MPM group; determining an intra prediction mode specified by the prediction mode index among the three intra prediction modes in the MPM group as the intra prediction mode of the current prediction unit when the mode group indicator indicates the MPM group; and deriving the intra prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes in the MPM group when the mode group indicator does not indicate the MPM group, wherein if the prediction mode Index is greater than or equal to a first mode in the MPM group, the value of the prediction mode index is increased by one; if the prediction mode index is greater than or equal to a second mode in the MPM group, the value of the prediction mode index is increased by one; and then if the prediction mode index is greater than or equal to a third mode in the MPM group, the value of the prediction mode index is increased by one, wherein the MPM group includes two non-directional modes and vertical mode when only one of the intra prediction modes of the left and the above blocks is available and the available intra prediction mode is one of the two non-directional modes, wherein the two non-directional modes are planar mode and DC mode, wherein the first mode is an intra prediction mode of a lowest mode number in the MPM group, the second mode is an intra prediction mode of a middle mode number in the MPM group, and the third mode is an intra prediction mode of a highest mode number in the MPM group, wherein the three intra prediction modes in the MPM group are sorted in an order of the first mode, the second mode and the third mode; and wherein, when only one of the intra prediction modes of the left and the above blocks is available and the available intra prediction mode is one of directional modes, the MPM group includes the two non-directional modes and the available intra prediction mode.