When read image data is mapped outside the range of a page memory, the invention allows a coordinate value to be stored in the page memory to be mapped after being shifted into the page memory, thereby allowing efficient utilization of the page memory, and in order to achieve the above-described object, the invention employs the configuration that when an integral number coordinate value (X.sub.int, Y.sub.int) is out of the range of an image memory, an X-coordinate conversion circuit and a Y-coordinate conversion circuit shift the coordinate value so that it comes in the range and outputs a coordinate value (XINT, YINT) and this shifting process allows read image data outside the image memory to be shifted into the area of the image memory to be stored therein and when image data stored in the image memory is read out, the coordinate is read out in reverse on the assumption that a shifted origin coordinate O (X.sub.smin, Y.sub.smin) serves as the coordinate origin of the image memory and the origin coordinate O (X.sub.smin, Y.sub.smin) is automatically stored in a register when the reading of the image is completed.