To provide a cap nut that can quickly cope with the case that protrusion lengths of bolt axes are different from one another, and can be used for a cap and a nut which are formed of synthetic resins.

A circular flange (11C) is integrally formed at the seat-side face of the nut (11), a male screw (11B) is formed at the external peripheral face of a flange, the cap (12) is formed into a shape which wholly covers the nut, a female thread (12A) is formed at the lower end internal face of the cap, and the cap is fixed to the nut by making the female thread of the cap and the male screw of the nut seat-face side flange screwed to each other. Preferably, the cap may be formed into a spherical shape, and the lower end of the cap may be pressed against a seat face of a member or a washer when fixed to the nut. Furthermore, the cap may be formed of a synthetic resin material.