1. A smoothing method comprising the steps of: (a) extracting 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; (b) judging black and white of the target pixel according to a predetermined rules and binarizing pixel data within the evaluation window; and (c) generating 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 outputting a multi-value correction signal in which the correction value is converted into a multi-value.