An endoscope system, comprising: an image signal acquisition unit configured to acquire a first image signal corresponding to a first wavelength band where an amount of light absorption changes according to oxygen saturation of blood hemoglobin, a second image signal corresponding to a second wavelength band where the amount of light absorption changes according to a blood volume of an observation target, a third image signal corresponding to a third wavelength band where a change in the amount of light absorption with respect to the oxygen saturation is small compared with the first image signal and a change in the amount of light absorption with respect to the blood volume is small compared with the second image signal, and a fourth image signal corresponding to a fourth wavelength band, a difference between a center wavelength of the first wavelength band and a center wavelength of the fourth wavelength band being 20 nm or more and 100 nm or less; and an oxygen saturation calculation unit configured to calculate oxygen saturation of the observation target for each pixel based on the first image signal, the second image signal, the third image signal, and the fourth image signal, characterized in that the endoscope system further comprises: a signal ratio calculation unit configured to calculate, for each pixel, a first signal ratio that is a ratio of the first image signal to the second image signal, a second signal ratio that is a ratio of the third image signal to the second image signal, and a third signal ratio that is a ratio of the second image signal to the fourth image signal, wherein the oxygen saturation calculation unit is configured to calculate the oxygen saturation based on the first signal ratio, the second signal ratio, and the third signal ratio. an image signal acquisition unit configured to acquire a first image signal corresponding to a first wavelength band where an amount of light absorption changes according to oxygen saturation of blood hemoglobin, a second image signal corresponding to a second wavelength band where the amount of light absorption changes according to a blood volume of an observation target, a third image signal corresponding to a third wavelength band where a change in the amount of light absorption with respect to the oxygen saturation is small compared with the first image signal and a change in the amount of light absorption with respect to the blood volume is small compared with the second image signal, and a fourth image signal corresponding to a fourth wavelength band, a difference between a center wavelength of the first wavelength band and a center wavelength of the fourth wavelength band being 20 nm or more and 100 nm or less; and an oxygen saturation calculation unit configured to calculate oxygen saturation of the observation target for each pixel based on the first image signal, the second image signal, the third image signal, and the fourth image signal, a signal ratio calculation unit configured to calculate, for each pixel, a first signal ratio that is a ratio of the first image signal to the second image signal, a second signal ratio that is a ratio of the third image signal to the second image signal, and a third signal ratio that is a ratio of the second image signal to the fourth image signal, wherein the oxygen saturation calculation unit is configured to calculate the oxygen saturation based on the first signal ratio, the second signal ratio, and the third signal ratio.