An endoscope system 10 includes: an image acquiring unit 54 that acquires an endoscope image obtained by imaging an observation target; a baseline information calculating unit 82 that calculates baseline information (operation value “Z”) by using the endoscope image, the baseline information being information about light scattering characteristics or light absorbing characteristics of the observation target and information that is at least not dependent on particular biological information; a region setting unit 83 that sets, by using the baseline information, a calculation region in which the index value is to be calculated, the index value indicating a state of the observation target; and an index value calculating unit 84 that calculates the index value in the calculation region set by the region setting unit 83, by using the endoscope image.