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Disclosed are lock nuts (100) and (100a) for preventing a leg bolt (4) from loosening from a leg assembly supporting electric home appliances, and a wrench (500) for loosening or tightening the lock nuts (100) and (100a′). The lock nuts (100) and (100a′) includes a hub having a screw hole (111), a plurality of ribs extended in a radius direction from a circumferential surface of the hub, and having sides (125) and (125a′) over which the wrench (500) or a finger is hooked, and supplementary member (130) provided allowing the wrench or finger to access form the radius direction of the hub to the sides (125) and (125a′), and coupling the ribs (120). The wrench (500) includes a long grip having at least one projection (550) and (560) on an outer circumferential surface thereof, and a box end (530) or an open end (520) provided at either of both ends thereof.