An image processing device performs an error diffusion on multilevel input image data to generate multilevel output image data that has fewer levels than the multilevel input image data. The image processing device includes a plurality of setting devices, each of which sets a threshold value to be used at the error diffusion, a threshold value selecting device that randomly selects one of the plurality of setting devices with respect to each pixel to be processed and allows the selected setting device to set a threshold value with respect to the pixel to be processed when the image processing device performs the error diffusion, and a converting device that converts the pixel to be processed into multilevel output image data with fewer levels than multilevel input image data by the error diffusion, in accordance with the threshold value set by the setting device selected by the threshold value selecting device. The plurality of setting devices includes at least a first setting device and a second setting device that sets a threshold value which is higher than a threshold value set by the first setting device.