The present invention accurately reproduces an image pattern by referring to neighboring pixels upon determining the threshold value of a target pixel so that a delay in dot generation can be prevented while emphasizing an image transition portion so that both graininess and sharpness are improved in the image. To this end, a neighboring pixel density difference detection part obtains an absolute value of the density difference between a target pixel and its neighboring pixels and selects the density of a neighboring pixel with a large absolute value as a reference pixel density. Further, a threshold value quantity determination part determines a threshold value corresponding to the density selected by the neighboring pixel density difference detection part. Then, a quantization part outputs an output value based on a comparison between the threshold value and a corrected value obtained by adding a pre-calculated error value of neighboring pixels to an input value.