PURPOSE: To provide a nut of simple structure, low cost, certainity and repeatability by forming two overhang jaw parts which can be elastically displaced in axial directions by radially extended slits on a nut, and axially displacing the jaw parts by a screw to be locked.

CONSTITUTION: A radial slit 18 is formed in a circular member 12 having flat front and back surfaces 13, 14 and a screw 17 to form overhang jaw parts 12A, 12B of different axial thicknesses. The jaw parts 12A and 12B have different thicknesses from each other, so the thinner jaw part 12A is elastically flexible to 12B, the jaw part 12B side 20B of a through hole 20 is threaded, the 20A side is set as a screw insertion hole, and both jaw parts 12A, 12B are tightened by a locking screw 22. This device can thus be manufactured in simple structure and at low cost, it can be securely actuated, and its performance is unchangeable even after repeated use.