To estimate the length of a stent implanted in a blood vessel after implantation of the stent. A stent length estimation device 100 includes an implantation start position specifying means for receiving, from a user, a designation of an implantation start position of an aneurysm treatment stent which is formed by helicoidally braiding a plurality of metal wires and specifying the implantation start position of the stent on a three-dimensional blood vessel image which represents a three-dimensional shape of the blood vessel, an implantation direction specifying means for receiving a designation of an implantation direction of the stent from the user and specifying the implantation direction of the stent on the three-dimensional blood vessel image, a stent specification specifying means for specifying a diameter of the stent after expanded, a length of the stent after expanded, the number of wires of the stent, and a pitch length of the wires of the stent after expanded as a specification of the stent, and an implanted stent length calculating means for calculating a length of the stent which is implanted and expanded along with a blood vessel diameter on the basis of the specification of the stent specified by the stent specification specifying means and the blood vessel diameter of the blood vessel in the implantation direction specified by the implantation direction specifying means from the implantation start position specified by the implantation start position specifying means.