1. An endoscope system comprising: a light projecting device for projecting first illumination light of a first wavelength range and second illumination light of a second wavelength range different from the first wavelength range into a test subject body, wherein light absorption coefficient of blood hemoglobin varies with 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 calculating oxygen saturation levels of the test subject using the first image signal; an oxygen saturation image producing device for producing 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.