To provide a unit and a method for orthogonal transformation in which the number of operations involved directly in power consumption of a circuit is minimized in the operation process of orthogonal transformation.

Sum and difference operations are performed on specified elements of pixel data constituting an input image, operation results are temporarily stored selectively in a plurality of storage means (registers) and operation results read out selectively from these storage means are multiplied by corresponding orthogonal transform coefficients. Multiplication results are accumulated to obtain orthogonal transform operation results of the image data.