Image decoding method using intra prediction mode

A method for decoding image, the method comprising:

• generating a residual block;
• reconstructing an intra prediction mode group indicator and a prediction mode index of a current block, wherein the intra prediction mode group indicator indicates whether the intra prediction mode of the current block belongs to which intra prediction mode group among a first group or a second group, and the prediction mode index specifies intra prediction mode in an intra prediction mode group indicated by the intra prediction mode group indicator;
• constructing the first group including three intra prediction modes using valid intra prediction modes of left and top blocks of the current block;
• determining the intra prediction mode corresponding to the prediction mode index in the first group as the intra prediction mode of the current block when the intra prediction mode group indicator indicates the first group;
• generating a prediction block on the basis of the determined intra prediction mode of the current block;
• generating a reconstructed block using the residual block and the prediction block;
• and
• performing deblocking filtering on a reconstructed picture,
• wherein the residual block is generated by inversely quantizing quantization block which is derived by converting quantization coefficient information, and by inversely transforming the inversely quantized quantization block,
• when only one of the intra prediction modes of the left and top blocks of the current block is present, two intra prediction modes are added to the first group, wherein the two intra prediction modes are different from the only one present intra prediction mode and are determined based on the only one present intra prediction mode,
• wherein when the intra prediction mode of the left block is not equal to the intra prediction mode of the top block and the intra prediction mode of the left block and the intra prediction mode of the top block are planar mode and DC mode, the first group includes the intra prediction modes of the left and top blocks and a vertical mode.