To provide an image processing method that can properly correct a contrast.

The image processing method for a multi-value image includes a 1st arithmetic step (S202) where a luminance frequency distribution of a multi- value image is obtained, a 2nd arithmetic step (S203) where 1st upper and lower limits of the luminance frequency distribution and a distribution width being a difference between the upper limit and the lower limit are obtained, and contrast adjustment steps (S205 and S206) where 2nd upper and lower limits in response to the lower limit of the distribution width are set (S204) when the distribution width is lower than a prescribed distribution width lower limit and the contrast of the image is adjusted so that a gradation width between the 2nd upper and lower limits has a maximum gradation width of the multi- value image.