1. An orthogonal transformation apparatus wherein two-dimensional orthogonal transformation of pixel data is executed by a block unit of m×n pixels (m and n are natural numbers: m=n, or m≠n) through breaking down said two-dimensional orthogonal transformation into one-dimensional orthogonal transformation of a row direction and a column direction, said orthogonal transformation apparatus comprising: an input device to which said pixel data of one block is inputted sequentially; a group of one-dimensional orthogonal transformation devices constituted with a plurality of one-dimensional orthogonal transformation devices which perform one-dimensional orthogonal transformation of said pixel data; a memory device for storing one-dimensional orthogonal transformation data of one block; and a selector which selects either said pixel data that is inputted via said input device or said one-dimensional orthogonal transformation data that is stored in said memory device, and outputs said selected data to said one-dimensional orthogonal transformation devices, wherein said one-dimensional orthogonal transformation devices: generate said one-dimensional orthogonal transformation data by performing first one-dimensional orthogonal transformation processing simultaneously to a plurality of rows of m-pixels of said pixel data when said pixel data is inputted via said selector, and then store said one-dimensional orthogonal transformation data to said memory device; and also generate two-dimensional orthogonal transformation data by performing second one-dimensional orthogonal transformation processing to said one-dimensional orthogonal transformation data on a plurality of columns of said n-pixels when said one-dimensional orthogonal transformation data corresponding to said plurality of columns of said n-pixels that are stored in said memory device is inputted sequentially via said selector.