Abstract: A torque transmission unit according to the invention comprises a bolt (1) which has curved outer peripheral surfaces (3) with a peripheral outline consisting of circular arcs circumscribing the sides of a square (.alpha.), respectively, and a socket wrench (2) which has curved inner peripheral surfaces (4) with a peripheral outline consisting of circular arcs circumscribing the sides of a square (.beta.), respectively. The curved inner peripheral surfaces (4) of the socket wrench (2) are brought into contact with the curved outer peripheral surfaces (3) of the bolt (1) and slid thereon by rotating the socket wrench (2). As a result, clamping can be performed with the axis of rotation of the wrench (2) made to coincide with that of rotation of the bolt (1). By virtue of this structure, any corner portion of the bolt (1) is prevented from being rounded.