A smoothing method extracts an evaluation window including a target pixel of a multi-value image data in which tone is represented by a multi-value in units of pixels, and judges black and white of the target pixel according to a predetermined rule, and binarizes pixel data within the evaluation window. Further, the smoothing method generates a correction value based on collating binarized pixel data within the evaluation window and a look-up table which stores patterns in vicinities of the target pixel, and outputs a multi-value correction signal in which the correction value is converted into a multi-value.