provided is an image processing device for comparing a determination value adjusted by comparing the gradation values of pixels included in inputted image data and threshold values of a dither mask prepared in advance , and a corrected gradation value obtained by correcting the gradation value of the pixel by the diffusion error from an adjacent processed pixel ; adjusting a determination value ; generating dot data which indicate the presence or absence of the dot formation by applying error diffusion ; and reducing the width of the adjustment of the determination value used in determining the dot formation so that the width of the adjustment is smaller in comparison with non - edge pixels in the case that a pixel is determined to be an edge pixel for which the difference in gradation value in relation to an adjacent pixel is equal to or greater than a predetermined value .