The endoscope system includes: an image signal acquisition unit acquiring first image signal in first wavelength range where the amount of light absorption changes according to the concentration of yellow dye, second image signal in second wavelength range where the amount of light absorption changes according to the blood volume of an observation target, and third image signal in third wavelength range where a change in the amount of light absorption according to the concentration of the yellow dye is smaller than the first wavelength range and a change in the amount of light absorption according to the blood volume is smaller than the second wavelength range； a signal ratio calculation unit calculating first signal ratio based on the first and second image signals and calculating second signal ratio based on the second and third image signals； and a warning notification unit calculating a threshold value and generates warning signal.
