We have many small diameter pipes that are cooling pipes for atomic power stations, boiler pipes, and gas or water pipe lines. They must be periodically inspected in order to protect the accident previously. Diameters of these pipes are different at the place where pipes change from the main to the branch. The inspection robot for these pipes must move different diameter. We propose a mobile robot that can surely move in a pipe whose diameter changes. The robot is constructed by the five rubber bellows as pneumatic actuator, six electromagnetic valves and six air feeding tubes. The bellows composes three somites by being arranged in two rows and three columns. The somite can make the arc like the bimetal by giving different pressure to each bellows. The arc somite can hold the pipe. The movement of the microrobot with three somites is imitating as for the accordion movement that is the movement of the snake operation. The fabricated mobile robot was confirmed to move in different diameter pipes whose diameters are between 46 mm and 60 mm. Its speed was 8.2 mm/s.