To securely apply torque to the tool engagement part of a tightening device without causing slide of an engagement port.SOLUTION: A tightening tool A for rotating a freely rotatable, regular polygonal cross-sectional tool engagement part B1 of a tightening device B comprises: an engagement port 1 having an open shape at its leading end, in which the tool engagement part is fitted so as to be freely detached. The engagement port has: a slide part 1a disposed in contact with a first tool engagement surface B3 of the tool engagement part so as to be slidable; an abutting part 1b disposed counter to a second tool engagement surface B4 of the tool engagement part; and a projection 1c projecting toward a recess B5 formed in the second tool engagement surface. The projection enters the recess while the engagement port is fitted on the tool engagement part.SELECTED DRAWING: Figure 1
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