An image forming apparatus includes: a printing device, a detection device, a processor, and a storage device. The printing device prints, with concentration corresponding to each input value, an image on a recording medium for the each input value. The detection device detects a concentration value of the each image. The storage device stores each variation information previously set for the each input value. The each variation information indicates a specific concentration value and a range of variation from the specific concentration value. The processor converts, of the concentration values detected by the detection device, the concentration value included in the corresponding variation range into the corresponding specific concentration value. The processor, based on the each concentration value obtained through the conversion process, generates a new gamma correction table.