An image processing device which generates a halftone image includes: an image size adjustment unit which adjusts a size of the input image; and a halftone processing unit which performs halftone processing on the input image size-adjusted to generate a halftone image, wherein: the image size adjustment unit adjusts the input image to a same size in two or more printing modes among the plurality of printing modes, and the input image of the same size is subjected to the halftone processing; the unit limits arrangement of dots constituting the halftone image to dot arrangeable places of a selected printing mode among the plurality of printing modes, and performs the halftone processing based on an error diffusion method using an error diffusion coefficient matrix according to the selected printing mode; and the functions performed by the image size adjustment unit and the halftone processing unit are achieved using a computer.