PURPOSE: To reduce computational quantity without using a picture signal decoded in a spatial area by converting a signal subjected to orthogonal- transformation at every area of first size into second size, and subjected orthogonal-inverse-transformation to this signal.

CONSTITUTION: When an output picture for which is processed in enlargement or reduction is required, a block size converting device 16 functions by switching a switch 17 on a lower side. Namely, an inverse-quantized signal by an inverse quantizer 12 is inputted to the block size converting device 16, and the enlargement or the reduction processing of block size is performed. Then, the enlargement or the reduction processing of the block size in the block size converting device 16 is executed for the coefficient of orthogonal transformation executed in the unit of block. Namely, at the time of the enlargement processing of the block size, a block is enlarged from m×m to m×n, and the higher order component of the transformation coefficients is added with, for instance, '0', and n×n inverse orthogonal transformation is executed for n×m blocks obtained in this way, and an enlarged picture is obtained.