PURPOSE: To strongly join metallic thin sheets by the device of a simple structure by providing the spring which energizes the die main body upwards in case of deforming by upsetting overlapped plural metallic thin sheets with a punch and die.

CONSTITUTION: A bottomed cylindrical projecting part is formed by pressing two sheets of metallic thin sheets 1a, 1b by a punch 3 in their overlapping state and by pushing into the die hole 2 of the die main body 4 consisting of plural divided pieces 4aW4d. The projecting part becomes in integral structure by bulging in the radial direction inside the upsetting pocket 2a when reaching to a stationary pressure receiving face 6a and the metal thin sheets 1a, 1b are strongly joined. Finishing the upsetting the die main body 4 is pressed upwards by the restoring force of a spring 10. The divided pieces 4aW4d which compose the die main body 4 are moved to the upper part along the inner wall face 8 of the supporting member with expanding along the radial direction by the opening of an expansion spring 12. In this way the projecting part is picked out of the upset pocket 2a.