An encoding controlling unit 3 selects one transformation block size which provides an optimal degree of encoding efficiency from a set of transformation block sizes which are determined in accordance with an encoding mode 7, and includes the transformation block size selected thereby in optimal compression parameters 20a to notify the transformation block size to a transformation/quantization unit 19, and the transformation/quantization unit 19 divides an optimal prediction differential signal 13a into blocks having the transformation block size included in the optimal compression parameters 20a, and carries out a transformation and quantization process on each of the blocks to generate compressed data 21.