To prevent a connection part for connecting eyeglasses parts from being loosened even at a rotating part.

A locking nut has a nut base (12) and a nearly truncated-cone-shaped or nearly dome-shaped extending projection (18) arranged in a direction of a shaft hole (30) for a screw of the nut base. The extending projection is provided with at least one or more slits (40) in parallel with the direction of the shaft hole for a screw on a wall surface (14) of the extending projection. A valley part of a screw thread inside the shaft hole for a screw of the extending projection is formed to be shallower than a valley part of a screw thread of the nut base, in addition, is formed to gradually become shallower as it approaches an edge part. When a bolt is inserted, locking effect is provided by elastic return force after elastic deformation of the nearly truncated-cone-shaped or nearly dome-shaped edge part of the extending projection, which makes the nut used for the connection part between the eyeglasses parts.