1. An intra prediction information encoding method 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 represents to which of the first group and a second group the intra prediction mode of the current block belongs, and a method of encoding the prediction mode index varies depending on the intra prediction mode group indicator.