In recent years, as changing the habit of eating, the pathology in the colon grows up annually. The colonoscopy is generalized, but if requires much time to acquire a dexterous skill to perform an operation and the procedure is painful to the patient. biomedical and robotic researchers are developing a locomotive colonoscope that can travel safe1y in colon. In this paper, we propose a new actuator and concept of semi-autonomous colonoscope. The micro robot comprises camera and LED for diagnosis, steer- ing system to pass through the loop, pneumatic actuator and bow-shaped flexible supporters to control a contact force and to pass over haustral folds in colon. For locomotion of semi-autonomous colonoscope, we suggest an actuator that is based on impact force between a cylinder and a piston. In order to validate the concept and the performance of the actuator, we carried out the simulation of moving characteristics and the preliminary experiments in rigid pipes and on the colon of pig.