There is provided an image processing device which converts multi-gradation pixels configuring a multi-gradation image of image data to be used in a color image forming apparatus, into unit dither patterns. The image forming apparatus includes photosensitive members on which developer images are formed by supplying developer to electrostatic latent images, and an exposure device which exposes the photosensitive members. The developer images on the photosensitive members are transferred to a transferred medium which is moved while contacting the photosensitive members. The image processing device includes a processing unit which converts a multi-gradation pixel of a first color into unit dither patterns, each unit dither pattern consisting of a first line extending in a predetermined direction and a second line extending in a direction intersecting with the first line when a gradation value of the multi-gradation pixel is within a predetermined range.