To provide a method to sense certainly a screw slacking and a slack sensor assuring simple mounting and an easy re-mounting and a slack detector.

A nut is provided with a mark to show the position in the rotating direction, and a reference member having a reference piece is installed on a bolt whereon the nut is engaged. The mark is a groove, dint, or a projection at the surface of the nut, and it may also be acceptable that an indicating member formed from a metal piece etc. is attached to the nut and a mark is formed thereon. The reference member has a base formed from a fastening member in a clip shape and is fitted with the reference piece confronting the mark. The base is attached to the bolt with a strength not easily making a rotational movement, in such a way as capable of being removed as appropriate. This allows sensing the nut slack through sensing of the displacement of the ocular mark from the reference piece.