PURPOSE: To strongly join metallic thin sheets by a simple device by employing a side opening method on the die main body in case of deforming overlapped plural metal thin sheets by upsetting by a punch and die.

CONSTITUTION: Two sheets of metallic thin sheets 1a, 1b are pressed to the upper end face of the main body 4 of a die by pressing it by a punch 3 in the overlapping state. A bottomed cylindrical projecting part 8 is further formed by bulging the pressure receiving part of the metal thin sheets 1a, 1b in the pressure direction by increasing the pushing in stroke of the punch 3 and pushed into a die hole 2. This die main body 4 is composed of the two pieces of movable die pieces 4a, 4b which are energized in closing direction always by a pressure spring 7, and the projecting part 8 bulges in the radial direction of the die hole 2 against the energizing of the movable die pieces 4a, 4b when reaching to the still pressure receiving face 5 at the upper end of an axial pressure receiving member 6. In this way the projecting part 8 of the metal thin sheets 1a, 1b is swaged in an integral structure and the metal thin sheets 1a, 1b are strongly joined.