Colonoscopy is an important medical action to detect disorders like colon cancer. However generally it is difficult to insert a scope into the colon, because the colon is flexible and complex shape. This study aims at development of an actuator which can add propelling ability to a colonoscope. We focus on rubber pneumatic actuators, because advantages of them, for example, high compliance, low cost, water proof, and so on, agree with the required properties of medical devices. In this paper, we design a novel rubber actuator consisting of three air chambers by nonlinear FEM (finite element method) and fabricate the actuator employing the design by extrusion molding method. Basic characteristics of the actuator are clarified by a motion capture system, and functional motion leading assisting colonoscope insertion is observed. Actually using dummy endoscope, transportation ability by the actuators is confirmed experimentally and insertion experiments into a large intestine phantom are conducted.