A processor for an endoscope system including an endoscope that projects first illumination light of a first wavelength range, and second illumination light that is white light of a second wavelength range different from the first wavelength 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, said processor comprising: a signal receiving means for receiving the first and second image signals from said endoscope; and an image producing means for producing a subject image of the test subject from the second image signal; characterized by an oxygen saturation calculating means (84, 85, 86) for calculating oxygen saturation levels of the test subject using the first image signal; and an oxygen saturation image producing means for producing an oxygen saturation image by changing color properties of the subject image according to the calculated oxygen saturation levels; wherein said 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; and an image color processing means that determines, with reference to said color changing amount memory, those color changing amounts corresponding to the oxygen saturation levels calculated by said 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, and wherein the color changing amounts are constant regardless of the oxygen saturation levels in a range above a given oxygen saturation level, so as to be displayed in biologically natural colors, and the color changing amounts increase or decrease depending on the oxygen saturation levels in a range below the given oxygen saturation level. a signal receiving means for receiving the first and second image signals from said endoscope; and an image producing means for producing a subject image of the test subject from the second image signal; characterized by an oxygen saturation calculating means (84, 85, 86) for calculating oxygen saturation levels of the test subject using the first image signal; and an oxygen saturation image producing means for producing an oxygen saturation image by changing color properties of the subject image according to the calculated oxygen saturation levels; wherein said 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; and an image color processing means that determines, with reference to said color changing amount memory, those color changing amounts corresponding to the oxygen saturation levels calculated by said 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, and wherein the color changing amounts are constant regardless of the oxygen saturation levels in a range above a given oxygen saturation level, so as to be displayed in biologically natural colors, and the color changing amounts increase or decrease depending on the oxygen saturation levels in a range below the given oxygen saturation level. 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; and an image color processing means that determines, with reference to said color changing amount memory, those color changing amounts corresponding to the oxygen saturation levels calculated by said 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, and wherein the color changing amounts are constant regardless of the oxygen saturation levels in a range above a given oxygen saturation level, so as to be displayed in biologically natural colors, and the color changing amounts increase or decrease depending on the oxygen saturation levels in a range below the given oxygen saturation level.