1. A method of encoding intra prediction information, comprising: determining an intra prediction mode of a current block; constructing a first group including three intra prediction modes using valid intra prediction modes of left and top blocks of the current block; determining an intra prediction mode group indicator and a prediction mode index of the current block; and encoding the intra prediction mode group indicator and the prediction mode index, wherein the intra prediction mode group indicator indicates whether the intra prediction mode of the current block belongs to the first group or a second group, and a method of encoding the prediction mode index varies depending on the intra prediction mode group indicator.