To provide a nut for theft-prevention which can be fastened without employing a special tool and which is hardly loosened after being fastened.

The nut for theft-prevention comprises a nut body formed of a cylindrical part having a spiral groove engraved in an inner peripheral surface and of a first serration blocking one opening in the cylindrical part and containing a through-hole, a turning body formed of a shaft part passing through the through-hole and a second serration, a cylinder body freely fitted to an outer periphery of the nut body and a lock body provided to a tip of the shaft body and made larger than a hole diameter of the through-hole. The nut body is provided with a first lock part becoming a lock part during rotation in one direction only. A surface facing the nut body of the turning body is provided with a second lock part locked in the first lock part provided to the nut body and turning the nut body in one direction only.