PURPOSE: To provide a device for varying the width of the opening part of a fastening in such a way as to be in conformity with the width of a desired nut or bolt head part.

CONSTITUTION: Plural mouthpieces 4, each of which advances/recedes in the center direction of a rotating shaft 1 and each inner face comes in contact with the outer face of a nut or a bolt head part N are disposed at the lower end of the rotating shaft 1 communicated with a rotating handle H. A cam cylinder 8 rotated along the outer periphery of the rotating shaft 1, with the inner peripheral surface 8a coming in contact with the back side of each mouthpiece 4 is provided, and the inner peripheral surface 8a of the cam cylinder 8 is formed into such cam curved face that the spacing of the respective mouthpieces 4 is changed while the respective mouthpieces 4 always maintain equal distance from the rotational center in association with the displacement of the angle-of-rotation of the cam cylinder 8. The opening part can be thereby varied to the width conformed to the square shape width of the desired nut or bolt by simple operation such as only rotating the cam cylinder 8 so as to contribute to the efficiency and stability of fastening/unfastening work.