PURPOSE: To provide a gray correcting function by generating a gray pattern by varying a specific black dot range of a bit pattern to a dot range consisting of black bits and white bits at a specific ratio.

CONSTITUTION: The dot pattern generating device 8 expands dot pattern corresponding to character codes inputted from an input device 2 in a line buffer memory 6 to generate dot patterns, character by character. A dot pattern correcting device 9 reads the dot pattern corresponding to a specific character out of the buffer memory 6 and performs dot correction processing for a dot pattern stored in a work memory 7 so that while the white dot part is left as it is, the black dot range is varied to the dot range obtained by dividing the black dot range into black dots and white dots at a specific ratio. Consequently, the gray pattern whose black dots are eventually thinned out is generated and stored in the line buffer memory 6. An output device 4 outputs the generated dot pattern or gray pattern.