1. A halftone dot image display unit comprising: a multi-tone level image display section for displaying a multi-tone level image represented by multi-tone level image data consisting of an assembly of pixels each having a pixel value according to a gradation; a position designation section for designating a desired position on the multi-tone level image displayed by said multi-tone level image display section; a data deriving section for deriving partial multi-tone level image data representative of the partial multi-tone level image within a peripheral area of the position designated by said position designation section, of the multi-tone level image, from the multi-tone level image data representative of the multi-tone level image displayed by said multi-tone level image display section; a partial halftone dot image data generation section causing the partial multi-tone level image data derived by said data deriving section to be affected by a predetermined threshold matrix so as to generate partial halftone dot image data representative of the partial halftone dot image associated with the partial multi-tone level image; and a partial halftone dot image display section for displaying a partial halftone dot image represented by the partial halftone dot image data generated by said partial halftone dot image data generation section.