PURPOSE: To obtain image output with high resolution and high output without missing an output graphic image by providing a magnification circuit in a graphic output system, reducing the processing quantity of a plotting processor, and performing interpolation.

CONSTITUTION: A plotting command string from a host computer 1 is stored in a command memory 2 under the management of a managing processor 3, and when the storage of the command string of one page in an image output device 10 on the memory 2 is completed, the processor 3 sends the command to the plotting processor 4. The processor 4 executes the interpretation of the command, and develops and stores by a dot unit a graphic image in a plotting memory 5. The processor 3 supplies the plotting command string being stored in the memory 2 to the processor 4 sequentially at every completion of the processing of one plotting command processing, and the processor 3 drives the magnification circuit 6 and an interpolation circuit 7, and magnifies the graphic image on an image memory 8, and also, stores it by interpolating a dot.