1. An image processing apparatus comprising: a contour extracting unit which extracts a contour pixel forming a contour part from pixels of image data having multiple gradation values; a contour processing unit which performs adjustment to reduce a gradation value of the contour pixel extracted by the contour extracting unit; a dithering unit which binarizes, by using a blue noise dither matrix, the gradation value of the contour pixel subjected to the adjustment by the contour processing unit; and a binarizing unit which binarizes, of the pixels of the image data, a gradation value of a non-contour pixel which is other than the contour pixel.