PURPOSE: To provide the device which shortens a processing time and saves the memory for bit image processing.

CONSTITUTION: This device consists of a reference line extraction part 12 which extracts run length data on a reference line corresponding to a conversion rate and lines before and after it from run length data inputted from outside, a black pixel counting part 13 which counts the totals of black pixels on the respective lines inputted from the reference line extraction part 12, a comparator 14 which compares the totals of the black pixels of the respective lines and decides the line having the largest number of black pixels, a selector 15 which selects the line having the largest number of black pixels according to the decision result of the comparator 14 and outputs it to a next stage, a conversion part 16 which converts the run length data into run length data corresponding to the reduction/enlargement conversion rate in the main scanning direction, a conversion table 17 in which the run length data corresponding to the conversion rate and error data generated at the time of the conversion are written, and an adder 18 which adds the error data sent out of the conversion part 16 and the run length data supplied from a selector 15 successively together.