It is decided whether a processing target block includes an edge in accordance with an edge detection result of the processing target block. In the case where the edge is included, it is then detected whether there is a flat portion. It is decided whether the processing target block includes the flat portion in accordance with a flat portion detection result. In the case where the flat portion is included, one of a first group of orthogonal transform sizes is selected. In the case where the edge is not included or the flat portion is not included, one of a second group of orthogonal transform sizes greater than the first group of orthogonal transform sizes is selected.