To make fastening by simple booster structure, without needing manufacturing accuracy, by constituting a torque-up mechanism of a tilted cam surface and a driven tilted cam surface, to use a needle bearing between both the surfaces.

In threadedly engaging the female screw 23, of the nut ring 11 of a fastening screw 10, with the male screw 20 of a driving spindle 19; since the set resistant loads of screws 20 and 23 are lighter than the contact resistant load of the inside of an operating ring 12, in an initial condition; the rotation of the operating ring 12 co-rotates to threadedly advance a nut ring 11. In this threadlike advance, when a flange ring 18 surface abuts on the side surface of a rotation tool 22 to stop the threadlike advance of the nut rin ring 11, the flange ring 18, a moderating ring 17, and a cam ring 15 stop their rotation by contact resistance. In this condition, when the operation ring 12 is fastened further approximately 30 degrees, a tilted cam surface 27 presses the driven tilted cam surface 31 of the cam ring 15 via a needle bearing 13, and this pressing force presses the flange ring 18 via the moderating ring 17, to firmly fasten a rotation tool.