An endoscope system comprising: a light source device configured to emit illumination light; an electronic endoscope; a processor that includes an image processing unit; and a monitor. The image processing unit includes: a detection unit configured to generate a first blood vessel detection image and a second blood vessel detection image; a separation unit configured to separate the first blood vessel detection image and the second blood vessel detection image into a shallow blood vessel image and a deep blood vessel image; a feature amount calculation unit configured to calculate a shallow blood vessel feature amount regarding the shallow blood vessel image and a deep blood vessel feature amount regarding the deep blood vessel image; and an evaluation unit configured to calculate severity of a lesion of a biological tissue by using the shallow blood vessel feature amount and the deep blood vessel feature amount.