To achieve the reliability of an optimum process at the time of butt- welding of sheet metal plates having different thicknesses when executing welding with a beam technique.

The process for butt-welding of the sheet metal plates by beam welding is provided in order to form a plane-shaped object having the different sheet metal thicknesses. Plural sheets of the sheet metal plate are collected to form a sheet metal stack 5. At least one end side of the sheet metal stack is aligned. Wear bars 4 and 6 are intervened in both sides on the side to be machined to clamp the aligned sheet metal stack. The aligned end side of the sheet metal stack is machined by a cutting tool 11. The machined ends of the sheet metal plates machined in the manner described above are brought into abutment on each other. The sheet metal plates are welded at the ends brought into abutment on each other.