In this paper, the authors report the results of an inspection test on a double-elbow pipe using a wave propagation type pipe inspection robot. In previous study, we developed a small and powerful robot to inspect pipes in the factory, and conducted an inspection test in a real field. In the test, it was confirmed the developed robot didn’t stably passage a double-elbow pipe (two 90-degree bent pipes installed close to 100-300 mm). It was confirmed that the path of the robot changed in the curved pipes and the displacement of the actuator was not effcientry converted into the amount of movement of the robot. In this study ,we increased the maximum extension length of the extension unit from 25 mm to 60 mm to solve the above problem. In the experiments of passage a double-elbow pipes in the lablatory, the redesigned robot was successfully pass the double-elbow pipes 10 times in a row.