PURPOSE: To obtain a printing image which is excellent in image quality by removing lacking of a dot at low magnification and unnaturalness of the image at high magnification by a method wherein a table data corresponding to enlarging on reducing magnification is selected among a plurality of sets of table data of which color patterns are differently established and brightness of each dot of a working graphic is determined.

CONSTITUTION: A CPU 2 calculates substantial manification based on dot data of an original graphic and a working graphic, and selects a table data preliminarily established following that. Then, the CPU 2 determines vertical and lateral dot positions of the working graphic, and calculates coordinates of the original graphic. What coordinates of the original graphic a point of the working graphic corresponds to is calculated, and a dot pattern by its peripheral four points is calculated. Successively vertical and lateral offset positions to the peripheral four points are calculated, a corresponding color pattern is selected from the table data, and a dot position is selected from the table data, and a dot position in the color pattern is determined. When the corresponding dot is 1 herein, a corresponding point of the working graphic is taken as black and when zero, it is taken as white.