To control the position of welding for a torch by detecting precisely if the position of the torch welding an inside surface has been properly arranged at a seam part when manufacturing a spiral steel tube.

A device is equipped with the seam position detecting device of a weld zone on the inside surface of a spiral steel tube, a temperature distribution detecting device 3, an arithmetic unit 4 performing the arithmetic of the shift quantities of a temperature peak position and a seam position and a controller 5 moving the position of a welding torch 7 so that the shift of the position becomes zero.