1. A contour extraction device that extracts a contour of an object using an active contour model, comprising: an image acquisition section that acquires a target image obtained by imaging a natural object; an initial shape setting section that sets an initial shape of an active curve for extracting the contour of the object appearing on said target image; an energy function setting section that sets an energy function, the energy function being expressed by a weighted linear sum of a plurality of kinds of energy terms defined correspondingly to a state of said active curve and being formulated so as to have a smaller value as approaching a shape of a contour to be extracted; an iterative computation section that minimizes said energy function by an iterative computation; an auxiliary function setting section that sets an auxiliary function formulated so as to change in accordance with the number of iteration times of said iterative computation; and an end timing judgment section that judges an end timing to end said iterative computation based on a transition of said energy function in the course of said iterative computation and a transition of said auxiliary function in the course of said iterative computation.