This invention decreases the count of access to a memory which stores image data regarding orthogonal transform, and quickly generates orthogonal transform coefficients. An apparatus includes a storage unit which stores image data, a memory controller which reads each block from the storage unit, a first transforming unit which receives and orthogonally transforms the input block, and calculates only one DC component, a selector which selects and outputs one of the block read out via the memory controller, and data of DC components from the first transforming unit that are equal in number to pixels included in the block, a second transforming unit which orthogonally transforms data that have been output from the selector and are equal in number to pixels included in the block, and outputs either one DC component or a plurality of AC components, and a controller which controls the memory controller, selector, and second transforming unit.