To obtain a tightening die which surely tightens and attaches any kind of lapped sheets, which is made firmly and also assembled easily, by including an anvil and plural numbers of forming elements mounted variably on the anvil in the die.

This die includes a base 12 and an anvil 10 having two supporting faces 14 which support forming elements 16 and which are inclined like a roof. In the forming element 16, its plane shape has a half-circular arc like projection 18 of projecting upward. When the material is displaced inside a space enclosed with the projections 18 and it is crushed between the action faces of the punch and the anvil, the material is made to flow laterally, the forming element 16 is deflected and the space for the flowing material is made to remain. Due to the inclination of the supporting face 14, the deflection motion of the forming element 16 is included with the composition to the lower side. Therefore, any kind of lapped sheet can be surely attached with tightening even if it has the remarkable toughness.