When dispersed large dots data associated with dispersed large dots which become visible as a result of making a copy or clustered small dots data (watermark pattern data) associated with clustered small dots which do not become visible as a result of making a copy is inputted to the watermark pattern generating section, the control section inputs the inputted dispersed large dots data or clustered small dots data to the watermark composition processing section. For a pixel corresponding to dispersed large dots data, the control section causes the watermark composition processing section to join data associated with an original image with data associated with a watermark and outputs obtained composite data to the image output apparatus. For a pixel corresponding to clustered small dots data, the watermark composition processing section outputs data associated with an original image to the image output apparatus.