PURPOSE: To obtain accurate tightening force by providing a circular recess on a surface contacting a member to be tightened and a groove at an outer periphery, by recognizing tightening condition by not being able to see the groove of the outer periphery after an upper section is buried into a lower section by a predetermined tightening force.

CONSTITUTION: A circular recess 1c having a diameter D, concentric to a central hole 1a for inserting a screw 2a, is provided on an end surface contacting a member 3 to be tightened, and a groove 1d is formed on an outer peripheral surface. A bottom surface 1e of the groove 1d is concentric to the recess 1c, a diameter D' is equal to the diameter D of the recess 1c, a thickness e of the groove is as near as the depth of the recess 1c, and a thickness of defining the minimum force necessary to tighten a screw bolt 2 to the member 3 to be tightened is rested between the recess 1c and the groove 1d. An area 1f section is cut at a predetermined value over a torque adding to a head 2b of the belt, and the groove 1d can not be seen thereby, so it is easy to confirm necessary tightening force.