To execute high-quality halftoning which does not generate a pseudo contour, periodicity, streak, shadow and the like.

There are provided 3 storage areas for a continuous tone image M 21, a halftone image B 22 and control data F 23, and initial values are assigned such that it is an original image for the continuous tone image M 21, arbitrary for the halftone image B 22 and 1 for all of the control data F 23. One pixel which is close to white or black is selected from among the pixels of the continuous tone image M 21 to be specified as a target pixel 11, and the control data F 23 corresponding to the pixel is set to 0. The target pixel 11 is quantized and the error resulted from the quantization is diffused to the pixels located around the target pixel 11. However, when pixels which are already quantized, in other words pixels of which control data are 0, are included therein, error is not diffused to these pixels and the error is shared among the rest of the pixels. A pixel which is close to white or black is designated at that point as the next target pixel 11.