PURPOSE: To prevent a dither image from varying in density and deteriorating in resolution by smoothing an enlarged or reduce dither image with smoothing matrixes which differ with scale factor and displaying the smoothed image.

CONSTITUTION: The enlarged or reduced dither image is stored in an image memory 11, read out in synchronism with the synchronizing signal of a display device, and smoothed with the smoothing matrixes 12-14. The smoothing process is carried out in pixel units by locally adding an attentional pixel and its peripheral pixels and the dither image after the smoothing process is selected by a selector 15 which uses the scale factor as selection information and displayed on a display device. For example, when the scale factor is ×2, the smoothing matrix 21 is selected, and P is set as the attentional pixel and locally added to its peripheral pixels according to the shape of the smoothing matrix 21. Further, the smoothing matrix 22 for a ×1 scale factor or the smoothing matrix 23 for a ×1/2 scale factor is selected and P is set as the attentional pixel similarly and locally added to the peripheral pixels according to the shape of the selected smoothing matrix.