An endoscope system comprising: a light source unit including one first light source configured to emit light including two color components with mutually different wavelengths and one second light source configured to emit light of a color other than the two color components; an image sensor having at least a first element part that has a spectral sensitivity for a first color component, a second element part that has a spectral sensitivity for a second color component out of the two color components of the first light source and a third element part that has a spectral sensitivity for the light of the color other than the two color components, wherein the image sensor has a range where the spectral sensitivity of the first element part and the spectral sensitivity of the second element part overlap each other; and a processor configured to image an observation object using the light emitted from the at least one first light source of the light source unit and obtain a first signal value of the first color component obtained in the first element part of the image sensor, a second signal value of the second color component obtained in the second element part and a third signal value of the light of the color other than the two color components obtained in the third element part of the image sensor, wherein the processor is configured to calculate a signal ratio between the first signal value and the second signal value, and set the signal ratio to a predetermined set value by changing at least one signal value out of the first signal value and the second signal value, and wherein the processor is configured to calculate a brightness value using at least one of the first signal value, the second signal value or the third signal value of the image sensor, specify a quantity of light of the first light source on the basis of the brightness value, and set the signal ratio to a predetermined set value by changing at least one signal value out of the first signal value, the second signal value and the third signal value in accordance with the quantity of light. a light source unit including one first light source configured to emit light including two color components with mutually different wavelengths and one second light source configured to emit light of a color other than the two color components; an image sensor having at least a first element part that has a spectral sensitivity for a first color component, a second element part that has a spectral sensitivity for a second color component out of the two color components of the first light source and a third element part that has a spectral sensitivity for the light of the color other than the two color components, wherein the image sensor has a range where the spectral sensitivity of the first element part and the spectral sensitivity of the second element part overlap each other; and a processor configured to image an observation object using the light emitted from the at least one first light source of the light source unit and obtain a first signal value of the first color component obtained in the first element part of the image sensor, a second signal value of the second color component obtained in the second element part and a third signal value of the light of the color other than the two color components obtained in the third element part of the image sensor, wherein the processor is configured to calculate a signal ratio between the first signal value and the second signal value, and set the signal ratio to a predetermined set value by changing at least one signal value out of the first signal value and the second signal value, and wherein the processor is configured to calculate a brightness value using at least one of the first signal value, the second signal value or the third signal value of the image sensor, specify a quantity of light of the first light source on the basis of the brightness value, and set the signal ratio to a predetermined set value by changing at least one signal value out of the first signal value, the second signal value and the third signal value in accordance with the quantity of light.