To provide a coil-bobbin type tool for friction stir welding which improves power without increasing the diameter of a stirring shaft or increasing the number of revolution of the shaft.

The tool 3 for friction stir welding is used for welding members to be welded each other by frictional heat by putting a welding part in which members to be welded are butted between an upper rotary body 11 and a lower rotary body 12 and moving a stirring shaft 24 which is rotated integrally between the upper rotary body 11 and the lower rotary body 12 along the welding part, wherein the stirring shaft 24 has a polyhedral shape where a plurality of stirring faces 241 parallel to the shaft direction are formed to the circumferential direction, and each stirring face 241 has a projecting curved surface.