To tighten a work to be tightened in a following manner even when the work is contracted by providing an energizing means to give the energizing force in the tightening direction on a nut to tighten the work to give the loosening preventive effect.

A hole part 5 is formed in the longitudinal direction from a tip of a bolt 4, and a coil spring 7 to accumulate the energizing force in the tightening direction is fitted to a bar-shaped operational rod 6 inserted in the hole part 5. In a tightening piece 1, after a nut 2 is screwed into the bolt 4 fitted to a work to be tightened, and tightened, the operational rod 6 is inserted in the hole part 5 of the bolt 4, a hexagonal nut 8 is locked to a detent part 5c, and then, the operational rod 6 is turned to the left. Then, the coil spring 7 is contracted, and the energizing force to turn the operational rod to the right direction is accumulated. In this condition, a locking part 6a of the operational rod 6 is locked to a locking recessed part 2a of the nut 2. Because the energizing force in the tightening direction is constantly applied to the nut 2, the nut 2 is turned following a column even when the column is dried and contracted, and the prescribed tightening force can be kept.