An endoscope system captures images of biological tissue when illuminated with first to third light, and generates first to third color image data. The amount of hemoglobin in the biological tissue and the oxygen saturation of the hemoglobin are calculated using components of the first to third color image data, an oxygen saturation distribution image that shows the distribution of the oxygen saturation is generated, and control is performed such that the generated oxygen saturation distribution image is displayed superimposed on a captured image of the biological tissue on a display. The oxygen saturation is calculated based on the amount of hemoglobin and a second ratio obtained using components of the color image data. The transparency is adjusted for pixels in which the value of the second ratio is outside an allowable range for the second ratio determined according to the amount of hemoglobin.