A friction stir welding tool (100, 100A, 100B, 100C, 100D) comprising: a shoulder member (7, 7C) having a shoulder surface for pressing a workpiece; a pin holder; and a stir pin (3, 3A, 3B) being rotatable about a first axis relative to the shoulder member (7, 7C), the stir pin (3, 3A, 3B) comprising: a base end portion held by the pin holder; a stirring portion; and an intermediate portion provided between the base end portion and the stirring portion and rotatable about the first axis together with the stirring portion, wherein the intermediate portion comprises a first portion and a second portion that is provided further in a first direction than the first portion, the first direction being defined as a direction from the base end portion toward the stirring portion, wherein the first portion comprises a protrusion protruding beyond an outer surface of the second portion in a direction away from the first axis, and wherein the protrusion has a surface that is pointed in the first direction and that defines a ring-shaped receiving surface that is configured to receive a material waste to be generated as a result of a friction stirring, wherein the shoulder member (7, 7C) has a first through hole which is provided at a leading end portion of the shoulder member (7, 7C) and through which the stir pin (3, 3A, 3B) is passable, and a second through hole through which the material waste entering inside the shoulder member (7, 7C) is dischargeable to an outside of the shoulder member (7, 7C), characterized in that the friction stir welding tool further comprises an adjustment mechanism located in the pin holder and configured to adjust a position of the ring-shaped receiving surface in a direction along the first direction so that a cross-section of the stir pin passing through an outermost edge of the ring-shaped receiving surface and perpendicular to the first axis crosses the second through hole, and the adjustment mechanism comprises a stopper member contactable with a base end surface (GS, 30s) of the base end portion, and a guide configured to guide a movement of the stopper member. a shoulder member (7, 7C) having a shoulder surface for pressing a workpiece; a pin holder; and a stir pin (3, 3A, 3B) being rotatable about a first axis relative to the shoulder member (7, 7C), the stir pin (3, 3A, 3B) comprising: a base end portion held by the pin holder; a stirring portion; and an intermediate portion provided between the base end portion and the stirring portion and rotatable about the first axis together with the stirring portion, wherein the intermediate portion comprises a first portion and a second portion that is provided further in a first direction than the first portion, the first direction being defined as a direction from the base end portion toward the stirring portion, wherein the first portion comprises a protrusion protruding beyond an outer surface of the second portion in a direction away from the first axis, and wherein the protrusion has a surface that is pointed in the first direction and that defines a ring-shaped receiving surface that is configured to receive a material waste to be generated as a result of a friction stirring, a base end portion held by the pin holder; a stirring portion; and an intermediate portion provided between the base end portion and the stirring portion and rotatable about the first axis together with the stirring portion, wherein the intermediate portion comprises a first portion and a second portion that is provided further in a first direction than the first portion, the first direction being defined as a direction from the base end portion toward the stirring portion, wherein the first portion comprises a protrusion protruding beyond an outer surface of the second portion in a direction away from the first axis, and wherein the protrusion has a surface that is pointed in the first direction and that defines a ring-shaped receiving surface that is configured to receive a material waste to be generated as a result of a friction stirring, wherein the shoulder member (7, 7C) has a first through hole which is provided at a leading end portion of the shoulder member (7, 7C) and through which the stir pin (3, 3A, 3B) is passable, and a second through hole through which the material waste entering inside the shoulder member (7, 7C) is dischargeable to an outside of the shoulder member (7, 7C), characterized in that the friction stir welding tool further comprises an adjustment mechanism located in the pin holder and configured to adjust a position of the ring-shaped receiving surface in a direction along the first direction so that a cross-section of the stir pin passing through an outermost edge of the ring-shaped receiving surface and perpendicular to the first axis crosses the second through hole, and the adjustment mechanism comprises a stopper member contactable with a base end surface (GS, 30s) of the base end portion, and a guide configured to guide a movement of the stopper member.