To easily carry out the adjustment of the part of a width-matching adjusting screw outside a space of a narrow width in which a bolt or nut is disposed when a tightening tool such as a monkey wrench is matched with such bolt or nut.

In a tightening tool in which a width-matching adjusting screw 1 is rotated to width-match mouthpieces 4 and 5 with the side face of the head part of a bolt or nut 5, a flex shaft 2 is extended in the rotation axis of said width-matching adjusting screw 1, and a rotary pinching part 3 which rotates the width-matching adjusting screw 1 is formed on the other end of the flex shaft 2. Furthermore, the flex shaft 2 is housed in a shaft guide hole 7 disposed along a gripping part 6 of the tightening tool, so that the rotary pinching part 3 at the end part of the flex shaft 2 is projected on this side nearer than the gripping part 6. Furthermore, a light source 8 is disposed on bolt or nut pinch- holding faces 4a and 5a of the mouthpiece 4 on the fixing side or the mouthpiece 5 on the sliding side.