PURPOSE: To perform good color separation, by converting optical images having the wide/narrow difference of wavelength bands into electric signals for the same original and setting the level of decision for color separation based on the variation factors of both signals.

CONSTITUTION: The image signals 11 and 12 delivered from the 1st and 2nd image sensors 6 and 9 are supplied to corresponding 1st and 2nd automatic background control circuits 13 and 14. Thus an image signal 15 is obtained at the white color light side is supplied to an arithmetic circuit 28 and a divider 29; while an image signal 18 of the cyan color light side is fed to the divider 29. If the result Va/Vb of division is 1, the read-out image information has no color. The image information is a cyan color group and a red color group in the case of Va/Vb<1 and Va/Vb>1 respectively. The result of division is binary coded with the prescribed value and supplied to the circit 28. The circuit 28 produces the red and black video signals 32 and 33 from the division information 31 and the signal 15 respectively.