An image forming apparatus includes a storage unit configured to store a divisor matrix including at least two kinds of divisor values, and a changing unit configured to change M (M>N) gradation image data into N (N>3) gradation image data, wherein the changing unit is configured to divide a pixel value of a target pixel of the M gradation image data by a divisor value corresponding to a pixel position of the target pixel to change the M gradation image data into the N-gradation image data.