To provide a screw driver tool for removing a rusted machine screw by driving an operation edge to the slope of a damaged concave part of a machine screw so as to bite in the slope even when the damaged concave part is in a mortar state.

At the tip of a driver shaft 2, rib projections 11 and valley grooves 12 with V-shaped cross section are alternately formed. Each rib projection 11 is formed into a trapezoidal shape in the cross section by the first valley surface 13 and the second valley surface 14 of the valley groove 12 and a rib peripheral surface 15. An operation edge 17 that bites a damaged concave part 7 is formed at the ridge line part of the first valley surface 13 and the rib peripheral surface 15. The operation edge 17 is tilted at a biting angle θ with respect to the center axis of the driver shaft 2, and the biting starting end 19 of the tip of the operation edge 17 when a machine screw is loosened is located upstream of the tail end 20 of the operation edge 17 in a loose rotation direction. During use, the driver shaft 2 is hammered toward the center axis, and the operation edge 17 disposed in the rib projection 11 is bitten into the damaged concave part 7 of the operation head 5 of the machine screw.