Abstract: An image memory is used for rotation of image data, and to shorten processing time. An image data rotating method uses an image memory capable of storing n×n pixels, and, for consecutive two pages in image data with the same size of a plurality of pages to be rotated, reads image data in a former page written in said image memory from each storage area at each address of said image memory in a predetermined order of reading according to the angle of 90 degrees to be rotated every k pixels for one line and every k lines for all lines; and writes image data in the latter page in the same storage area at the same address of said image memory in the predetermined order.