An image processing apparatus for sequentially processing image regions which overlap by a predetermined number of pixels in a predetermined direction, the apparatus comprising: a memory in which a first region, a second region, a third region, and a fourth region respectively store rectangular image regions, of which three out of four regions form a rectangular combined image region, wherein the predetermined number of pixels is assigned to widths of respective image regions in the second region, the third region, and the fourth region in the predetermined direction; processing means for performing the sequential processing of changing data stored in three out of the four memory regions, by alternating between a first processing in which the three memory regions are the first region, the second region, and the fourth region, and a second processing in which the three memory regions are the first region, the second region, and the third region; storage means for, before performing each alteration of the predetermined processing, storing data in the first region and the second region, and copying the data of one memory region by storing, for the first processing, in the third region, the same data as the data stored in the second region, and for the second processing, in the fourth region, the same data as the data stored in the second region; and selection means for, in the case of the first processing, selecting the first region, the second region, and the fourth region as memory regions to be processed by said processing means, and in the case of the second processing, selecting the first region, the second region, and the third region as memory regions to be processed by said processing means. a memory in which a first region, a second region, a third region, and a fourth region respectively store rectangular image regions, of which three out of four regions form a rectangular combined image region, wherein the predetermined number of pixels is assigned to widths of respective image regions in the second region, the third region, and the fourth region in the predetermined direction; processing means for performing the sequential processing of changing data stored in three out of the four memory regions, by alternating between a first processing in which the three memory regions are the first region, the second region, and the fourth region, and a second processing in which the three memory regions are the first region, the second region, and the third region; storage means for, before performing each alteration of the predetermined processing, storing data in the first region and the second region, and copying the data of one memory region by storing, for the first processing, in the third region, the same data as the data stored in the second region, and for the second processing, in the fourth region, the same data as the data stored in the second region; and selection means for, in the case of the first processing, selecting the first region, the second region, and the fourth region as memory regions to be processed by said processing means, and in the case of the second processing, selecting the first region, the second region, and the third region as memory regions to be processed by said processing means.