To provide an image processing method capable of executing stable gradation processing without deteriorating gradation reproducibility even when gradation reproducibility in a prescribed area is deteriorated.

The image processing method for converting a multi-gradation image into multi-valued quantized image by an error diffusion method is provided with a procedure S10 for storing or updating a quantized value in each quantization of a pixel and a procedure S7 for forcedly changing the quantized value of the pixel concerned to the outside of an equivalent range when the quantized value of the pixel is included within a previously set range (S4) and a quantized value of a pixel quantized and stored before the pixel concerned is in the equivalent range. Even when gradation reproducibility in a prescribed area is deteriorated, the deterioration of gradation reproducibility is not easily generated by mixing other density values and executing false gradation reproduction, so that stable gradation processing can be attained.