PURPOSE: To provide a nut fastener which is easily handled with simple constitution and by which a work process can be reduced and which can contribute to an increase in work speed and cost reduction.

CONSTITUTION: A nut fastener is composed of a fastening gear 11 which is divided into two parts and is composed of a bevel gear having a square hole 11c corresponding to a nut to be fastened, a gear holder 1 composed of a pair of gear cases 2 and 3 to respectively and rotatably hold one half part of the fastening gear 11 and a driving gear 12 which is rotatably supported with one of the gear cases and meshes with the fastening gear 11 and drives this in rotation. A pair of gear cases 2 and 3 are made openable and closable, and when they are opened, the fastening gear 11 can be divided into two parts.