An image forming method exposes image bearing members by simultaneously reflecting light beams corresponding to different colors by different reflection surfaces of a polygon mirror, transforms electrostatic latent images formed on the image bearing members into toner images for correction, transfers the toner images in an overlapping manner onto a transfer body, and calibrates overlapping positions of the toner images based on an optical detection of the toner images. The toner images are arranged at positions on the transfer body such that the toner images of different colors have no overlap therebetween, even if the toner images shift in a direction perpendicular to the transport direction due to a color registration error. Two first toner images simultaneously formed by two corresponding light beams reflected by one reflection surface of the polygon mirror are arranged adjacent to each other in a transport direction of the transfer body, and are sandwiched by two second toner images simultaneously formed by the two corresponding light beams reflected by the one reflection surface of the polygon mirror along the transport direction.