An apparatus for decoding an image, comprising: an entropy decoding unit configured to derive intra prediction information and quantized coefficient information from a bit stream received from an image encoding device; an inverse scanning unit configured to generate a quantized transform block by inversely scanning the quantized coefficient information; an inverse quantization unit configured to generate a transform block by inversely quantizing the quantized transform block; an inverse transform unit configured to generate a residual block by inversely transforming the transform block; an intra prediction unit configured to restore an intra prediction mode of a current prediction block based on the intra prediction information and to generate a prediction block according to the intra prediction mode; and an adder configured to generate an original block by adding the prediction block and the residual block, wherein if it is determined that a size of the transform block is larger than a predetermined reference size, the inverse scanning unit inversely scans the quantized coefficient information in the unit of a predetermined size of a subset using an inverse scan pattern determined according to the intra prediction mode, wherein the intra prediction unit restores an intra prediction mode group indicator and a prediction mode index of a current prediction block, generates an intra prediction mode group using available intra prediction modes of a left prediction block and an above prediction block neighboring the current prediction block, wherein the number of intra prediction modes belonging to the intra prediction mode group is fixed to three, and determines an intra prediction mode which is included in the intra prediction mode group and indicated by the intra prediction mode index as the intra prediction mode of the current prediction block when the intra prediction mode group indicator indicates the intra prediction mode group, wherein, when only one intra prediction mode out of the intra prediction modes of the left prediction unit and the above prediction unit is available, the intra prediction mode group includes the one available intra prediction mode of the respective prediction units and two additional intra prediction modes that are determined such that when the one available intra prediction mode is a non-directional intra prediction mode, the two additional intra prediction modes include another non-directional intra prediction mode and a vertical mode, wherein, when the one available intra prediction mode is a DC mode, the two additional intra prediction modes include a planar mode and the vertical mode, and wherein, when the one available intra prediction mode is a planar mode, the two additional intra prediction modes include a DC mode and the vertical mode. an entropy decoding unit configured to derive intra prediction information and quantized coefficient information from a bit stream received from an image encoding device; an inverse scanning unit configured to generate a quantized transform block by inversely scanning the quantized coefficient information; an inverse quantization unit configured to generate a transform block by inversely quantizing the quantized transform block; an inverse transform unit configured to generate a residual block by inversely transforming the transform block; an intra prediction unit configured to restore an intra prediction mode of a current prediction block based on the intra prediction information and to generate a prediction block according to the intra prediction mode; and an adder configured to generate an original block by adding the prediction block and the residual block, wherein if it is determined that a size of the transform block is larger than a predetermined reference size, the inverse scanning unit inversely scans the quantized coefficient information in the unit of a predetermined size of a subset using an inverse scan pattern determined according to the intra prediction mode, wherein the intra prediction unit restores an intra prediction mode group indicator and a prediction mode index of a current prediction block, generates an intra prediction mode group using available intra prediction modes of a left prediction block and an above prediction block neighboring the current prediction block, wherein the number of intra prediction modes belonging to the intra prediction mode group is fixed to three, and determines an intra prediction mode which is included in the intra prediction mode group and indicated by the intra prediction mode index as the intra prediction mode of the current prediction block when the intra prediction mode group indicator indicates the intra prediction mode group, wherein, when only one intra prediction mode out of the intra prediction modes of the left prediction unit and the above prediction unit is available, the intra prediction mode group includes the one available intra prediction mode of the respective prediction units and two additional intra prediction modes that are determined such that when the one available intra prediction mode is a non-directional intra prediction mode, the two additional intra prediction modes include another non-directional intra prediction mode and a vertical mode, wherein, when the one available intra prediction mode is a DC mode, the two additional intra prediction modes include a planar mode and the vertical mode, and wherein, when the one available intra prediction mode is a planar mode, the two additional intra prediction modes include a DC mode and the vertical mode.