To prevent a printing quality level from being reduced by preventing a blur or an isolated point from occurring at a boundary in a character and graphic data in an error diffusion method.

A difference between a pixel to be processed and pixel data of a pixel adjacent to the pixel is calculated in the error diffusion method. When the difference of the pixel data exceeds a certain threshold, the error value of the error diffusion method is reset. When the difference of the pixel data is less than the threshold, error data are added as usual. By the control, the blur or the isolated point is prevented from being generated in character data and a line drawing, so that the excellent printing quality level is obtained.