PURPOSE: To improve the precision for positioning and to improve the efficiency of manufacture by adopting plural numbers of rack and pinion mechanism for deciding the center of a steel tube.

CONSTITUTION: Restricting arms 6 to restrict an inner wall 7a of a steel tube 7 at three places of the circumference are provided movable to the radius direction inside a lower jig 2 positioning an inserted subject 8. When a central gear 3 is rotated and controlled, a pinion gear 4 is rotated, and the restricting arms 6 are pushed with a rack gear 5 to the steel tube inner wall 7a in being uniform in three directions. This positioning jig for welding can improve the precision for positioning the parts and can do a holding process until fixing of welding after welding while moving the parts by providing the restricting arm 6 being expandably controlled with a rack and pinion mechanism inside the lower jig 2 when the inserted subject 8 is welded to the steel tube 7.