An image forming apparatus includes setting means configured to set writing start address for each line of image data, based on information for correcting distortion of a scanning line, correction means configured to read the image data from storage means on a line-by-line basis in a second direction orthogonal to the first direction of the image, and execute correction processing to write the image data in the storage means from the writing start address set by the setting means on a line-by-line basis in the second direction of the image, output means configured to read the image data corrected by the correction means from the storage means on a line-by-line basis in the first direction of the image to output the image data.