To provide a method of and a device for machining a thin sheet metal where, in bevel machining for a thin sheet metal, distortion, warpage, and the generation of dust, chips or the like are eliminated, a surface treatment film at the part subjected to bevel machining is removed, and pinholes are not generated at the welded part of the bevel.

By a first die 10 in which a rectilinear or circular projecting part 32 is formed and a second die 12 in which a rectilinear or circular groove part 38 is formed, a mountain-shaped recessed part 40 is formed at the first face 18a of a thin sheet metal 18 by the projecting part 32. When the mountain-shaped recessed part 40 is formed, a swelling part 42 projecting to the groove part 38 formed at the second die 12 is formed at the second face 18b on the side opposite to the first face 18a in which the recessed part 40 is formed. By the formation of the swelling part 42, distortion is not generated at the thin sheet metal 18. When the thin sheet metal 18 is cut at the positions passing through the recessed part 40 and the swelling part 42, the wall face of the mountain-shaped recessed part 40 is made into a tilted face 46, the tilted face 46 is made into a bevel face, and welding of the two thin sheet metals 18, e.g., at an angle of 90 degrees is performed.