An image forming apparatus forms a color image using image carriers for a plurality of colors by electrophotography. The image forming apparatus stores, for each color, information on the shift amount between an ideal scanning line and an actual scanning line in the main scanning direction on the image carrier. For each color, a color misregistration correction amount calculation unit converts a readout address in a bitmap memory (403), based on the stored shift amount information, and reads out image data. A color misregistration correction unit (404) performs tone correction for the readout image data based on the address, outputting image data containing the value of the decimal part. A multi-level error diffusion unit (405) performs quantization by multi-level error diffusion processing for the tone-corrected image data containing the value of the decimal part.