Abstract: An image processing apparatus in which an original image read by a scanner is rotated by an angle of 90.degree. to be output. One-page image data output from the scanner is stored in a memory on a dot-by-dot basis. The memory is adapted to read therefrom and write therein dot data along a dot array and along a line array. A first read-out operation in which the dot data are successively read out of the memory along the dot array and a second read-out operation in which the dot data are successively read out of the memory along the line array are alternately switched every time reading a one-page image of the document original is competed. Dot data newly generated is written in a storage area from which the previous dot data has been read out at substantially the same time when the previous dot data is read out of the memory.