1. An endoscope system comprising: a light projecting device for projecting a first illumination light of a first wavelength range and a second illumination light of a second wavelength range different from the first wavelength range into a test subject body, wherein a light absorption coefficient of blood hemoglobin varies with an oxygen saturation thereof in the first wavelength range; an image signal capturing device using an imaging device to capture a first image signal from the first illumination light as reflected from inside the test subject body and a second image signal from the second illumination light as reflected from inside the test subject body; an image producing device for producing a subject image of the test subject from the second image signal; an oxygen saturation calculating device for numerically calculating oxygen saturation levels of the test subject using the first image signal; a gain memory for memorizing oxygen saturation levels in association with gains for changing pixel levels of the subject image according to the oxygen saturation levels; an image color processing device that determines, with reference to said gain memory, the gains corresponding to the oxygen saturation levels calculated by said oxygen saturation calculating device, and the pixel levels of the second image signal being multiplied by the determined gains to produce an oxygen saturation image by changing color properties of the subject image according to the calculated oxygen saturation levels; and a display device for displaying the oxygen saturation image, wherein the second illumination light comprises white light, and the subject image comprises an ordinary image captured under the white light, wherein the gains are constant regardless of the oxygen saturation levels in a range above a given oxygen saturation level in the ordinary light image, and wherein the gains increase or decrease depending on the oxygen saturation levels in a range below the given oxygen saturation level in the ordinary light image.