To prevent tampering release of nuts.

A tamperproof cover 1 for nuts uses a cover body 5 shaped into a frustum of a cone, which defines an internal nut holding space 2 and has a top projection 6 integrated via a constriction 7 and a tapped hole 8 passing through the ceiling of the nut holding space 2. With the cover body 5 placed over a nut 4 in which a bolt 3 is threaded, the top projection 6 is turned with wrenches or others to tighten the cover body 5 whose internal tapped hole 8 is threaded onto the bolt 3. Even after the cover body 5 is locked up, the top projection 6 is further turned to apply torque to the constriction 7 until it breaks off. The cover body 5 left over the nut 4 protects it against direct wrenching. The tapped hole 8 engaging the bolt 3 protects the cover body 5 against uplift or removal. The outer conical surface keeps wrenches or others slipping on the cover body 5 or trying in vain to turn and dislodge it.