1. An image encoding method comprising: determining an intra prediction mode of a current prediction unit; encoding the current prediction unit using the intra prediction mode; constructing a first group including three intra prediction modes using valid intra prediction modes of left and top blocks of the current prediction unit; determining an intra prediction mode group indicator and a prediction mode index of the current prediction unit; encoding the intra prediction mode group indicator and the prediction mode index, and wherein the intra prediction mode group indicator indicates whether the intra prediction mode of the current prediction unit 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.