To improve welding precision and working efficiency, in welding angle steels at right angle to each other, by using a jig in which two L-shaped bolsters are joined so as to be slidable on a base in which two sides are vertically adjacent.

On the entire length of the vertically adjacent two sides of the base 1 of the jig, a groove 11 is formed whose cross-section is trapezoidal with the width on the bottom which is larger than that of the open part; also, the orthogonally crossing point of the two adjacent sides is designed to be greatly missing. Meantime, the two outer sides of the L-shaped bolster 2 is formed with the groove 21 on the entire length; also, the horizontal side facing the base 1 is provided with a projected rib 22 whose cross section is the same as the of the groove 11 of the base, so that the bolster is slidable when inserted into the groove 11. In welding angle steels 3, the projected rib 22 of the bolster 2 is inserted into the groove 11 of the base 1 to assemble the jig. Thus, one side of the angle steel 3 is inserted into the groove 21 on the outer horizontal side of the bolster 2 structured at right angle, with two angle steels 3 abutted on each other. Further, another angle steel is inserted into the groove 21 of the vertical side of this bolster 2, and abutted on the horizontal angle steels 3.