To reduce redundancy in processing period, when a block with pixels whose number is lower than n by n is subjected to orthogonal transformation.

In the case of a block 300 at a corner, only regions of effective pixels excluding a pixel supplement region are written into a storage section, and only the regions of the written effective pixels are read out from the storage section. Then, after a first one-dimensional orthogonal transformation is performed, the result of transformation is also written back into the addresses which is read out in the storage section. Thereafter, as a direction in which a one-dimensional orthogonal transformation is to be performed, the pixel data is read out with the order of horizontal and vertical directions changed in the order, and a two-dimensional orthogonal transformation is performed. At this time, the result of transforming the effective pixel region, such as the read-out in the one-dimensional orthogonal transformation.