An endoscope system (10, 100, 120, 200) comprising: an illuminating section for illuminating a subject; an image signal obtaining section for receiving and imaging reflection light from a subject illuminated by the illuminating section, to obtain a first image signal, a second image signal, and a third image signal, the first image signal corresponding to first reflection light including a first wavelength range in a blue region, an extinction coefficient varying in accordance with an oxygen saturation level of hemoglobin in blood in the first wavelength range, the second image signal corresponding to second reflection light including a second wavelength range in a red region, the extinction coefficient varying in accordance with the blood volume in the second wavelength range, the third image signal being used for standardizing the first and the second image signals, the third image signal corresponding to third reflection light including a third wavelength range in a green region, the third wavelength range differing from the first and the second wavelength ranges; an image generator having an oxygen saturation calculator for obtaining the oxygen saturation level not dependent on the blood volume, based on the first to the third image signals, and an oxygen saturation image generator for producing an oxygen saturation image based on the oxygen saturation level obtained by the oxygen saturation calculator, the oxygen saturation level being visualized in the oxygen saturation image; and a display section for displaying the oxygen saturation image, wherein the image signal obtaining section images the subject, illuminated with blue narrowband light, with a color image sensor to obtain the first image signal, and the image signal obtaining section images the subject, illuminated with white light, with the color image sensor to obtain the second and the third image signals, further comprising: a standardization signal obtaining section for obtaining a first standardization signal and a second standardization signal, the first standardization signal being obtained by standardizing the first image signal with the third image signal, the second standardization signal being obtained by standardizing the second image signal with the third image signal wherein the image generator uses a correlation between a first color information and a combination of the first and second standardization signals to produce the oxygen saturation image. an illuminating section for illuminating a subject; an image signal obtaining section for receiving and imaging reflection light from a subject illuminated by the illuminating section, to obtain a first image signal, a second image signal, and a third image signal, the first image signal corresponding to first reflection light including a first wavelength range in a blue region, an extinction coefficient varying in accordance with an oxygen saturation level of hemoglobin in blood in the first wavelength range, the second image signal corresponding to second reflection light including a second wavelength range in a red region, the extinction coefficient varying in accordance with the blood volume in the second wavelength range, the third image signal being used for standardizing the first and the second image signals, the third image signal corresponding to third reflection light including a third wavelength range in a green region, the third wavelength range differing from the first and the second wavelength ranges; an image generator having an oxygen saturation calculator for obtaining the oxygen saturation level not dependent on the blood volume, based on the first to the third image signals, and an oxygen saturation image generator for producing an oxygen saturation image based on the oxygen saturation level obtained by the oxygen saturation calculator, the oxygen saturation level being visualized in the oxygen saturation image; and a display section for displaying the oxygen saturation image, wherein the image signal obtaining section images the subject, illuminated with blue narrowband light, with a color image sensor to obtain the first image signal, and the image signal obtaining section images the subject, illuminated with white light, with the color image sensor to obtain the second and the third image signals, further comprising: a standardization signal obtaining section for obtaining a first standardization signal and a second standardization signal, the first standardization signal being obtained by standardizing the first image signal with the third image signal, the second standardization signal being obtained by standardizing the second image signal with the third image signal wherein the image generator uses a correlation between a first color information and a combination of the first and second standardization signals to produce the oxygen saturation image. a standardization signal obtaining section for obtaining a first standardization signal and a second standardization signal, the first standardization signal being obtained by standardizing the first image signal with the third image signal, the second standardization signal being obtained by standardizing the second image signal with the third image signal wherein the image generator uses a correlation between a first color information and a combination of the first and second standardization signals to produce the oxygen saturation image.