1. An image coding method of selecting a processing size of orthogonal transformation on a block-by-block basis and coding a target block by the selected processing size, the image coding method comprising: calculating a histogram of pixel values included in a first target block included in an original image; determining whether or not the calculated histogram is bimodal; detecting a size of a plane region included in the first target block; determining whether or not the detected size of the plane region is equal to or greater than a first threshold value; and selecting, as the processing size of the orthogonal transformation for the first target block, (i) a first processing size when it is determined that the histogram is bimodal and it is determined that the size of the plane region is equal to or greater than the first threshold value, and (ii) a second processing size greater than the first processing size when it is determined that the histogram is not bimodal or it is determined that the size of the plane region is smaller than the first threshold value.