To obtain a compact unit accurately regulating a bolt fastening force and wherein a spring constant is increased with an increase in deformation amount.

A detection seat for bolt fastening force is composed of an upper washer 1, a ring 2, a lower washer 3, elastic elements 5 incorporated in these, and a washer 4. The fastening force is regulated by a clearance G between the upper surface of the elastic element 5 and the upper surface of the ring 2, and when the fastening force exceeds the regulated value, the bending amount of the elastic element 5, the upper washer 1 and the lower washer 3 can cope with the excessive fastening force.