The snake model, which is well known in the domain of image segmentation, suffers from the problem that the result is heavily sensitive to the initialization, though many remedies (pressure force, gradient vector flow, etc.) have been proposed. In this paper, a new external force, called the adaptive pressure force, which can alternate adaptively between inflation and shrinkage according to the position of the contour, is presented. It can solve the problem of initialization very effectively. Besides, with the new external force, the problems of leakage of weak edges and poor convergence to concave boundaries can also be solved. Experiments demonstrate its effectiveness and robustness.