1. An intra prediction encoding control method used in intra prediction encoding having a plurality of prediction modes and prediction block sizes, wherein the prediction mode and the prediction block size each can be switched when encoding is performed, the method comprising: a step of computing a flatness degree index which indicates a degree of flatness of a target area for encoding; a step of determining a prediction block size used in intra prediction encoding of the target area, in accordance with the computed flatness degree index; and a step of selecting a prediction mode used in the intra prediction encoding of the target area, for the determined prediction block size.