A processor of an endoscope system uses components of color image data of biological tissue illuminated by at least two types of light to generate an oxygen saturation distribution image of hemoglobin in the biological tissue and to calculate a certainty of an oxygen saturation. Furthermore, the processor controls a display mode of the oxygen saturation distribution image according to the degree of the certainty. The certainty drops if the value of a first component a of the color image data is lower than a lower limit threshold value. The certainty drops if the value of a second component b of the color image data is greater than an upper limit threshold value. The lower limit threshold value and the upper limit threshold value are values of two different pieces of color image data.
