In the development of a microrobot for inspecting the inside of pipes with an internal diameter of 1 to 3 inches, the author of this paper designed an in-pipe mobile mechanism adaptive to pipe diameter based on a snake robot and evaluated it theoretically and experimentally. The authors fabricated 2 robots of different size. The first, Prototype A, was used to verify functions and confirm the driving mechanism. It demonstrated basic motion and moved in all directions during an experiment in which the robot passed through a T-shaped pipe. The second, Prototype B, was downsized and could run in narrow channel whose width changes from 0.7 to 3.9 inches.