Abstract: A nut comprises a metal member including a nut body having an internally threaded portion formed therein and an extension formed on one end of the nut body so as to surround an opening of the internally threaded portion, the metal member being subjected to surface treatment; and a ring member made of a synthetic resin, having an outer peripheral portion united with an inner side of the extension and having a ring inner diameter substantially equal to a minor diameter of the internally threaded portion, the outer peripheral portion being united with the inner side of the extension by insert-molding using the metal member as an insert; wherein the extension has at least one recess formed in an inner portion, and the outer peripheral portion has a rotation-preventing portion fit into the at least one recess, the rotation-preventing portion being formed by the insert molding.