PURPOSE: To prevent two colors from being printed in an overlapping state and enhance printing quality, by a system wherein when the output condition of a first color and that of a second color for a dot picture element at the same address overlap with each other, an image pattern in a non-preferred color is corrected so as to inhibit outputting of the non-preferred color, and the corrected pattern is stored.

CONSTITUTION: A character generation controller (CGC) 24A reads character pattern data and printing position data from a global-local memory (GLM) 28, and develops the data into dots on a second image map memory (IMM) 212, and stores the dots into the memory (IMM) 212. By access to a first IMM 211 with the same address, an overlay dot pattern is read, and when the output condition of a first color (red) and that of a second color (black) for a dot picture element at the same address overlap with each other, namely, when a dot pattern in the first color and dot pattern in the second color overlap with each other, the data for the overlapping part is corrected so as to inhibit the outputting of the non-preferred first color (red), namely, corrected to be '0', and the corrected data is rewritten into the first IMM 211.