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, by using Formula (1) or (2): act = ∑ i = 0 N x ⁢ ⁢ ∑ j = 0 N y ⁢ ⁢ ( s ⁡ ( i , j ) - 〈 s 〉 ) 2 Formula ⁢ ⁢ ( 1 ) act = ∑ i = 0 N x ⁢ ⁢ ∑ j = 0 N y ⁢ ⁢  s ⁡ ( i , j ) - 〈 s 〉  Formula ⁢ ⁢ ( 2 ) where s(i,j) indicates the pixel value at position (i,j) in the target area for encoding, and <s> indicates an average of the relevant pixel values; 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; and in the step of determining the prediction block size: one or more thresholds having different values 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.