PURPOSE: To improve the maintenance property through the detection of position and direction of a running body quickly and surely, by providing an attitude detecting means for the running body, in a welding line inspection of a pressure vessel of an atomic power generating plant.

CONSTITUTION: A running body 9 is run along a track provided along the wall of a construction and attitude detection mercury switches SW22 and 23 are provided. A top 22c of the SW22 is tilted left by about 45 degrees from the progressing direction of the running body 9 and a top 23c of the SW23 is tilted to the right from the progressing direction of the body 9 by about 45 degrees. Thus, the SWs 22 and 23 are turned on or off depending on the left, downward and right attitude of the body 9. Then, the switching state of the SWs 22 and 23 is electrically detected and the direction of the body 9 is surely detected. Further, a fixed mark is provided at each branch of the track of the body 9 in addition to the attitude detecting means and the mark detecting means is provided to the body 9, allowing to detect the attitude.