an image forming device scans an original document and generates a bi - level image signal . the image forming device obtains a first counted value by counting black pixels included in the bi - level image signal . the image forming device obtains a first generation copy by forming an image on a printing medium according to the bi - level image signal of the original document . the image forming device scans the first generation copy and generates a second bi - level image signal . the image forming device obtains a second counted value by counting the black pixels included in the second bi - level image signal . the image forming device compares whether or not the second counted value is within a target range of the first counted value , and controls the density of an image to be formed according to the comparison result .