To simplify fitting and manufacture a locking device at low cost by forming a male screw insert hole on the central top part of a plate spring bent to be angled.

A locking device 10 is an one-sheet angled plate spring and has a male screw insert hole formed in the central top part and gentle turned parts 12, 13 formed on both sides thereof. The material quality used is spring steel, but iron, stainless steel and the like may be used. In the case of fastening members A, B with a screw by the device 10, a long bolt 15 having a fixture 14 formed at one end is fitted to the member B by a bolt 16 and a nut 17, the device 10 is mounted on the long bolt 15 inserted in the bolt insert hole 18, and when a nut 19 is fastened, the device 10 is spread laterally to apply repulsive force to the nut 19. The plate spring is spread at the turn-down parts 12 without damaging the surface of the member A, so that the repulsive force keeps the nut 19 from being loosened by a little vibration. Accordingly, loosening between the members can be surely prevented in a simple structure.