An endoscope system processor includes: a blood vessel region determination unit that obtains a likelihood of a blood vessel region by a numerical value on the basis of a shape characterizing a blood vessel in each of a plurality of parts of an image of a biological tissue, from the image obtained by an electronic endoscope; and a blood vessel feature amount calculation unit that calculates a blood vessel feature amount indicating a feature amount of the blood vessel region by integrating the numerical values of the likelihood of the blood vessel region with each other in the entire image. The blood vessel region determination unit calculates an approximation degree as the likelihood of the blood vessel region, using a spatial filter.