1. An image processing apparatus comprising: a two-dimensional orthogonal transform unit configured to perform a two-dimensional orthogonal transform on a plurality of images; an one-dimensional orthogonal transform unit configured to perform a one-dimensional orthogonal transform in a direction in which the images are arranged on two-dimensional orthogonal transform coefficient data obtained by performing the two-dimensional orthogonal transform on the images by using the two-dimensional orthogonal transform unit; and a three-dimensional orthogonal transform coefficient data encoder configured to encode three-dimensional orthogonal transform coefficient data obtained by performing the one-dimensional orthogonal transform on the two-dimensional orthogonal transform coefficient data by using the one-dimensional orthogonal transform unit.