A processor for an endoscope system including an endoscope that projects first illumination light of a first wavelength range and second illumination light of a second wavelength range different from the first range into a test subject body, and captures a first image signal and a second image signal respectively from the first illumination light and the second illumination light as reflected from inside the test subject body, wherein light absorption coefficient of blood hemoglobin varies with oxygen saturation thereof in the first wavelength range, the processor comprising: a signal receiving means for receiving the first and second image signals from the endoscope; an image producing means for producing a subject image of the test subject from the second image signal; an oxygen saturation calculating means (84, 85, 86) for calculating oxygen saturation levels of the test subject body with respect to every pixel of the subject image using the first image signal; characterized in that an oxygen saturation image producing means for producing an oxygen saturation image by changing color properties of a corresponding pixel of the subject image according to the oxygen saturation level of each pixel calculated by the oxygen saturation calculating means, characterized in that the oxygen saturation image producing means comprises: a color changing amount memory for memorizing oxygen saturation levels in association with color changing amounts served for changing pixel levels of the second image signal according to the oxygen saturation levels, the color changing amounts being gains by which the pixel level of each pixel of the second image signal is multiplied, and wherein the gains are changed such that the gains are set to "1" in a case where the oxygen saturation levels are equal to or more than a threshold and the gains linear decreases for the red signal and linear increases for the green and the blue signal in a case where the oxygen saturation levels are less than the threshold; an image color processing means that determines, with reference to the color changing amount memory, the color changing amount corresponding to the oxygen saturation level of each pixel calculated by the oxygen saturation calculating means, and changes the pixel levels of the second image signal individually by the determined color changing amounts, to produce the oxygen saturation image. a signal receiving means for receiving the first and second image signals from the endoscope; an image producing means for producing a subject image of the test subject from the second image signal; an oxygen saturation calculating means (84, 85, 86) for calculating oxygen saturation levels of the test subject body with respect to every pixel of the subject image using the first image signal; a color changing amount memory for memorizing oxygen saturation levels in association with color changing amounts served for changing pixel levels of the second image signal according to the oxygen saturation levels, the color changing amounts being gains by which the pixel level of each pixel of the second image signal is multiplied, and wherein the gains are changed such that the gains are set to "1" in a case where the oxygen saturation levels are equal to or more than a threshold and the gains linear decreases for the red signal and linear increases for the green and the blue signal in a case where the oxygen saturation levels are less than the threshold; an image color processing means that determines, with reference to the color changing amount memory, the color changing amount corresponding to the oxygen saturation level of each pixel calculated by the oxygen saturation calculating means, and changes the pixel levels of the second image signal individually by the determined color changing amounts, to produce the oxygen saturation image.