An endoscope has an elongated tube for entry in a body cavity. A guide assembly includes a shaft sleeve for mounting on the elongated tube. A housing sleeve is supported around the shaft sleeve. Three endless belts are secured to the housing sleeve, for endlessly moving in an axial direction of the elongated tube, and propelling the elongated tube by contacting a cavity wall of the body cavity. A driving device is contained in the housing sleeve, for driving the endless belts. Three curved walls constitute the housing sleeve, and support the endless belts movably and in a transversely curved state. Each of the endless belts includes an upper belt run, curved by the curved walls convexly outwards, for contacting the body cavity. A lower belt run is disposed inside, and tensioned between the driving device and the belt rollers.