An image forming apparatus which can be utilized by switching a plurality of image forming speeds, comprising: image forming means (120, 130, 140, 150) configured to form a density measuring image pattern on an image carrier (121, 131, 141, 151) at a first image forming speed; transfer means configured to transfer the image pattern onto a recording medium at the first image forming speed to form a density measuring image on the recording medium; reading means configured to read the density measuring image formed on the recording medium; determination means configured to determine density correction characteristics to be applied to correct density characteristics of said image forming means (120, 130, 140, 150) and said transfer means, based on the reading result obtained by said reading means; holding means (28, 29) configured to hold the density correction characteristics determined by said determination means; measuring means (160, 28) configured to measure a density value of the image pattern formed on the image carrier (121, 131, 141, 151) by said image forming means (120, 130, 140, 150) at the first image forming speed by applying the density correction characteristics; reference density value storage means configured to store, as a reference density value, the density value of the image pattern measured by said measuring means (160, 28); and creation means configured to create modification data to modify the density correction characteristics, which are held in the holding means, for a second image forming speed, which is different from the first image forming speed, from a difference between a density value of an image pattern formed on the image carrier (121, 131, 141, 151) by said image forming means (120, 130, 140, 150) at the second image forming speed by applying the density correction characteristics, and the reference density value stored in said reference density value storage means. image forming means (120, 130, 140, 150) configured to form a density measuring image pattern on an image carrier (121, 131, 141, 151) at a first image forming speed; transfer means configured to transfer the image pattern onto a recording medium at the first image forming speed to form a density measuring image on the recording medium; reading means configured to read the density measuring image formed on the recording medium; determination means configured to determine density correction characteristics to be applied to correct density characteristics of said image forming means (120, 130, 140, 150) and said transfer means, based on the reading result obtained by said reading means; holding means (28, 29) configured to hold the density correction characteristics determined by said determination means; measuring means (160, 28) configured to measure a density value of the image pattern formed on the image carrier (121, 131, 141, 151) by said image forming means (120, 130, 140, 150) at the first image forming speed by applying the density correction characteristics; reference density value storage means configured to store, as a reference density value, the density value of the image pattern measured by said measuring means (160, 28); and creation means configured to create modification data to modify the density correction characteristics, which are held in the holding means, for a second image forming speed, which is different from the first image forming speed, from a difference between a density value of an image pattern formed on the image carrier (121, 131, 141, 151) by said image forming means (120, 130, 140, 150) at the second image forming speed by applying the density correction characteristics, and the reference density value stored in said reference density value storage means.