To improve image quality without producing line unevenness even when the image with dither pattern for each pixel is magnified by non-integer times.

When an unmagnified image I1 is magnified, each pixel P2 of the magnified image I2 is created by repeating gradation data of each pixel P1 of the unmagnified image I1 by pixel. Then, exceptional part of the number of repeating the gradation data is discriminated based on the condition for repeating the gradation data, adjacent M pixels (M is an integer of two or more) out of a plurality of pixels P2 which constitute the magnified image I2 is used as a processing unit and the processing unit is determined to be the exceptional part or not. For the processing unit determined not to be the exceptional part, the gradation data of each pixel P2 is converted into the gradation data expressed by presence of dot formation by the processing unit, while for the processing unit determined to be the exceptional part, the gradation data of the pixel P2 is converted into the gradation data expressed by presence of dot formation by pixels whose number is smaller than the number of the processing unit.