PURPOSE: To measure parameters which represent actual tightening tension well with accuracy and good reproducibility although an assembly element is inexpensive, small and easy to use.

CONSTITUTION: This assembly element has symmetrical axes X, X' and two faces 3, 4. A force sensor 5 symmetrical in shape is bonded to the first face 3 of the element 1 and has the same symmetrical axes X, X' as the assembly element. Preferably, the force sensor 5 consists of a ceramic support member 8, and a central metallic layer 9 and an annular metallic layer 10 are affixed to one side thereof and joined to each other by an electrically conducting layer 11 of high sensitivity to deformation.