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, wherein in the intra prediction encoding, a predicted signal is generated using peripheral pixels; in the step of determining the prediction block size: one or more thresholds having different values, which are stored in a threshold table, are set in accordance with selectable prediction block size so as to set a plurality of level ranges for the flatness degree index, and different block sizes are assigned to the level ranges in a manner such that the higher the degree of flatness of the target area, the larger the determined prediction block size for the encoding, in the step of computing the flatness degree index, the target area is divided into a plurality of divided areas, the flatness degree index is computed for each divided area and the flatness degree index of the target area is computed based on the flatness degree indexes of the divided areas, the intra prediction encoding control method is characterized in that: the flatness degree index of the target area is set to the maximum value among the flatness degree indexes of the divided areas, and the flatness degree index of each of the divided areas is computed as an L1 dispersion of pixel values of each of the divided areas. 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, wherein in the intra prediction encoding, a predicted signal is generated using peripheral pixels; 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, wherein in the intra prediction encoding, a predicted signal is generated using peripheral pixels; in the step of determining the prediction block size: one or more thresholds having different values, which are stored in a threshold table, are set in accordance with selectable prediction block size so as to set a plurality of level ranges for the flatness degree index, and different block sizes are assigned to the level ranges in a manner such that the higher the degree of flatness of the target area, the larger the determined prediction block size for the encoding, one or more thresholds having different values, which are stored in a threshold table, are set in accordance with selectable prediction block size so as to set a plurality of level ranges for the flatness degree index, and different block sizes are assigned to the level ranges in a manner such that the higher the degree of flatness of the target area, the larger the determined prediction block size for the encoding, in the step of computing the flatness degree index, the target area is divided into a plurality of divided areas, the flatness degree index is computed for each divided area and the flatness degree index of the target area is computed based on the flatness degree indexes of the divided areas, the intra prediction encoding control method is characterized in that: the flatness degree index of the target area is set to the maximum value among the flatness degree indexes of the divided areas, and the flatness degree index of each of the divided areas is computed as an L1 dispersion of pixel values of each of the divided areas. the flatness degree index of the target area is set to the maximum value among the flatness degree indexes of the divided areas, and the flatness degree index of each of the divided areas is computed as an L1 dispersion of pixel values of each of the divided areas.