Provided are an image processing device for computing an index representing a change over time of observation object blood vessels, an endoscope system, and an image processing method. An image processing device (65) is provided with an image acquiring unit (81) for acquiring a plurality of endoscope images of an observation object captured and obtained at mutually different times by an endoscope, a blood vessel extraction unit (82) for extracting observation object blood vessels from each of the plurality of endoscope images, a blood vessel information computing unit (83) for computing a plurality of blood vessel information for each of the blood vessels extracted from the endoscope images, a blood vessel parameter computing unit (84) for computing a blood vessel parameter relating to the blood vessels extracted from the endoscope images by calculation using the blood vessel information, and a blood vessel variation index computing unit (85) for computing a blood vessel variation index (108) representing a change over time of the blood vessels using the blood vessel parameter.
