In the case where an output value of a document reading section for at least any of R, G and B is out of a range between a first reference value and a second reference value due to a change in an output value of a reading device caused by an electrical noise, a change of supply voltage, etc. when a document is being read, a second color conversion table is created in accordance with the above change by expanding ranges in a first color conversion table, which ranges are around predetermined first and second reference values and included in regions of RGB color system associated respectively with predetermined tone values of Y, M and C. Using the second color conversion table thus obtained, a color conversion is carried out from the RGB color system into the YMC color system.