PURPOSE: To set up a bogie without reference to a flat or curved working plane and corresponding to a change of curvature also by setting up the bogie on a working plane through belts for a running guide and running this along those belts with a regular or reverse revolution of a driving wheel.

CONSTITUTION: A bogie 3 is arranged to the position nearby a weld line of a pipe 1, and the bogie 3 is set up to the pipe 1, pressed against the outside of the pipe by two steel belts 13 while the belts are fixed to the pipe 1, wound through the outsides of a driving wheel 11 and a driven wheel 12. The bogie 3 runs circumferentially on the outside surface of the pipe 1 along the belts 13 because the belts 13 is fixed to the pipe 1 when a motor 10 is started and the driving wheel 11 is revolved and driven. A detector 4 loaded to the bogie 3 is moved to the direction rectangular with the running direction of the bogie 3 by a locomotive mechanism 5. Thus, the bogie 3 does not fall even if the working plane is vertical, runs along the working plane with the regular or reverse revolution of the driving wheel 11 even if in the curved plane, and can correspond to a change of the pipe diameter by changing the lengths of the belts 13.