tone level s2 as the initial point as illustrated in fig6 d incidentally the highest density d 39 cmy in this case is displaced in proportion to the slope of the leading edge characteristic mentioned above in short the density characteristic assumes a leading edge characteristic such that the density in each tone level preceding the threshold tone level s2 remains at 0 whereas the density in each tone level following the threshold tone level s2 rises from the threshold tone level s2 and reaches the aforementioned degree d 39 cmy at the highest tone level in this case the ucr processing is not executed and the density in each tone level except for the tone preceding the threshold tone level s2 is represented with the three color cmy inks a density characteristic illustrated in fig6 e is obtained by overlapping the density characteristics illustrated in fig6 c and fig6 d what is aimed at by the present invention is attained by forming a lookup table in consideration of this density characteristics converting image data into dot modulating data with reference to this lookup table and then forming a color image on the basis of the dot modulating data to be specific the control device with necessary information collected from the lookup table formed in due consideration of the density characteristic mentioned above and the addresses of the image data executes the processing for converting the image data sequentially read out of the frame memory 4 in the form of rgb signal into dot modulating data in the form of cmyk signal s4 then the control device causes the dot modulating data resulting from the conversion at the step s4 to be sequentially stored in line units sorted by color in the.