An image forming system for suppressing the generation of moire effect of a print image, provided with a receiving apparatus for receiving an image data of a document, a printer controller for detecting a screen angle of a document image based on the image data, selecting from a plurality of dither patterns a dither pattern with a screen angle different from the detected screen angle and setting the dither pattern, and a printer for reproducing a quasi-halftone using the dither pattern set up by the printer controller.