PURPOSE: To improve working efficiency by strongly pressing the peripheral edges of steel products to form recesses and projecting parts for spot welding.

CONSTITUTION: One side edge of the plate steel products 2 is strongly pressed from both the front and rear sides by a pressing machine to form the recesses 3 for arc welding as grooves and the plate-shaped projecting parts 4 for brackets. The projecting parts 4 are then erected on a lower bent part 6a of a side plate 6. The part from the recesses 3 to the projecting parts 4 and the lower bent part 6a are spot welded by a depositing metal 7. The groove part can be formed by pressing and the welding is executed by utilizing the projecting parts 4 and the recesses 3 and therefore, the built-up of the deposited metal is prevented. The need for polishing the built-up metal after welding is thus eliminated and the working efficiency of welding is improved.