n (n is an integer that is equal to or greater than 2) error diffusion processing units are respectively provided to correspond to division images obtained by dividing an image into n images in a main scanning direction, and at least a part of halftone processing on two division images is performed simultaneously. When one error diffusion processing unit that processes the division image which is positioned ahead in the main scanning direction processes one of rasters up to an end portion in the main scanning direction to obtain diffusion errors that are diffused into pixels in the neighborhood of an objective pixel, the one error diffusion processing unit delivers a forward-direction diffusion error to the other error diffusion processing unit. Furthermore, other error diffusion processing unit that receives the forward-direction diffusion error starts the halftone processing on one raster, obtains the diffusion error in an end portion in an opposite direction, and then delivers an opposite-direction diffusion error that is a diffusion error which is used by the one error diffusion processing unit in the halftone processing of a subsequent raster, among the diffusion errors, to the one error diffusion processing unit.