The image forming apparatus includes first and second reading portions which read images of first and second faces of an original, an image forming portion which forms an image on a sheet based on image data read by the reading portions, a calculation portion which calculates difference between respective image data, a correction portion which calculates correction data to decrease difference of the image data based on the image data difference, and a control portion which controls the image forming portion so that an image for correction to correct the reading accuracy difference is formed on a sheet based on the correction data; and the calculation portion calculates difference between respective image data when the image for correction formed on a sheet is respectively read by the first and second reading portions, and the correction portion recalculates correction data based on the image data difference calculated by the calculation portion.