A reading section outputs density information of a gradation pattern image of a recording member. A gradation correction characteristic generation section generates a first gradation correction characteristic f(x). At first image quality adjustment, a detection section detects densities of patch pattern images on an image carrier, and the gradation correction characteristic generation section generates a second gradation correction characteristic g(x). At normal image quality adjustment, the detection section detects densities of patch pattern images on the image carrier, and the gradation correction characteristic generation section generates a second gradation correction characteristic g′(x). A calculation section calculates a difference gradation correction characteristic g″(x) between the gradation correction characteristic g(x) and the second gradation correction characteristic g′(x). An image quality maintaining control section multiplies the difference gradation correction characteristic g″(x) by the first gradation correction characteristic f(x), and adjusts an image quality of an image formed on the recording member.