PURPOSE: To reduce encoded data quantity by utilizing the correlation between same frequency components between adjacent blocks in the orthogonal transformation encoding system.

CONSTITUTION: The m×m (m<n) sets of transformation coefficient groups of a low frequency component or an AC component are outputted directly among n×n sets of transformation coefficients obtained from a discrete cosine transformation circuit 3. On the other hand, the transformation coefficient group of the high frequency component or a DC component is sub-sampled (interleaved) for each adjacent k×k block by a sub sample circuit 5. A transformation variable for a high frequency component of one block is used for a transformation variable for a high frequency component of other block among adjacent k×k blocks. Thus, the transformation coefficient only is used for the transformation circuit of the high frequency component coded actually. Thus, a data quantity of the encoded data of the high frequency component is reduced to 1/k2.