1. An image processing device comprising: first memory means storing a plurality of growth weight value matrix patterns which serve as the elements making up a screen and which have mutually different dot growth patterns; pattern selecting means selecting said growth weight value matrix patterns at random; screen generating means arranging growth weight value matrix patterns selected by said pattern selecting means and generating said screen; and image converting means taking input image data and using said screen generated by said screen generating means to perform conversion into halftone image data using first data that has been quantized based on weight values in said growth weight value matrix pattern and intermediate value data corresponding to a difference between a weight value and said image data.