To solve a problem of deterioration inherent to an error diffusing method that is encountered in a low-density area of lateral end of an image with a relatively simple method.

A pixel of image data represented by multi-valued gradation valued is binarized or converted to a predetermined plurality of values. When converting to the plurality of values, an error between the value after the target pixel is converted to the plurality of valued and an original value is found, and the error is distributed to other unprocessed pixel. When performing the error diffusion method, an appropriate lowest density value is previously set. When a density value of the first target pixel of image data scan line is below the lowest density value, the density value of the target pixel is changed to the lowest density value previously set, and the error diffusion processing is performed based on the changed density value.