Abstract— In wet condition of complex pipe, pipe inspection robots using fluidic flexible actuators have advantages because of no electrical leakage and easy traveling into pipe while deforming its body. In the previous study, a robot that can travel in a complex pipe with inner diameter of 100 mm by changing pipe holding position like an inchworm was proposed and tested. However, the pipe holding mechanism using extension type flexible pneumatic actuator could not hold the pipe surly because of its small contacting area with pipe. In this paper, to make sure to surely the hold pipe with smaller diameter of 75 and 50 mm, the development of a holding mechanism with large contacting area with pipe is described. The improved holding mechanism has similar construction of automobile tire wheel to decrease volume change for expanding. In addition, a thinner type sliding/bending mechanism using a novel extension type flexible pneumatic actuator for the pipe diameter of 50 mm is proposed and tested. The pipe inspection robots using both pipe holding mechanisms for the pipe diameter of 75 and 50 mm are also developed. The driving test of pipe inspection robots using both holding mechanisms is carried out. As a result, it can be confirmed that both robots can travel smoothly in a complex pipe that includes corner and tee joints.