gradation value is not used in the vicinity of a maximum value u201c 255 u201d due to the above described reasons there is a tendency that a part of a cubic ridge line in a rgb coordinate space among rgb respective gradation values at least two gradation values are u201c 0 u201d or u201c 255 u201d is not used in the rgb image data loaded by the scanner unit 100 therefore in reality there is often a case where pixels corresponding to the above described ridge line parts do not exist even in a case where they do exist they are small in number therefore if these pixels are changed in rgb gradation values there is hardly found a difference between an original manuscript image and a copy image further in the present embodiment the color conversion lut is embedded in a ridge line corresponding to yellow color which makes it more difficult to realize a difference in change in rgb gradation values specifically yellow color is less conspicuous in change in color and even if there is some change in rgb gradation values the color change is hardly noticeable due to the above reason in the copy processing of the present embodiment despite such change in rgb gradation values it is possible to obtain a high quality color image by reproducing an original manuscript image still further in the copy processing of the present embodiment it is not necessary to execute complicated processing in which the color conversion lut is changed during color conversion processing thus the color conversion processing.