A friction stir welding tool (200, 300, 400, 500) comprising: a stir pin (220, 320, 420, 520) comprising a rotation axis; and a housing (210, 310, 410, 510) comprising a surface (234a, 334a, 434a, 534a, 634a), wherein the housing (210, 310, 410, 510) contains the stir pin (220, 320, 420, 520) rotatably about the rotation axis relative to the housing (210, 310, 410, 510), wherein the housing (210, 310, 410, 510) comprises a housing hole opening on the surface (234a, 334a, 434a, 534a, 634a), wherein the stir pin (220, 320, 420, 520) comprises a stir portion that is located at one end of the rotation axis, that is contained in the housing hole with a part of the stir portion protruding through and beyond the surface (234a, 334a, 434a, 534a, 634a), and that is to be inserted into a workpiece, wherein the housing (210, 310, 410, 510) comprises a discharge hole (238, 338, 438, 538, 638, 639) through which the housing hole is connected to an outside of the housing (210, 310, 410, 510), wherein the housing hole comprises a first housing hole and a second housing hole that are connected to each other along the rotation axis, wherein the first housing hole is open on the surface (234a, 334a, 434a, 534a, 634a), wherein the second housing hole has a hole diameter larger than a hole diameter of the first housing hole, wherein the discharge hole (238, 338, 438, 538, 638, 639) is located at a position including a boundary between the first housing hole and the second housing hole, wherein the housing (210, 310, 410, 510) comprises a shoulder (230, 330, 430, 530, 630) and a housing body (212, 312, 412, 512) that are separate from each other, the shoulder (230, 330, 430, 530, 630) comprising the surface (234a, 334a, 434a, 534a, 634a) and the discharge hole (238, 338, 438, 538, 638, 639), wherein the shoulder (230, 330, 430, 530, 630) comprises an attached portion (232, 332, 432, 632) that is attached to the housing body (212, 312, 412, 512), wherein the housing body (212, 312, 412, 512) comprises a holding portion (217, 317, 417) that holds the attached portion (232, 332, 432, 632), and wherein the shoulder (230, 330, 430, 530, 630) includes a connection position where an inner surface of the discharge hole (238, 338, 438, 538, 638, 639) and an inner surface of the second housing hole are connected to each other in the rotation direction of the stir pin (220, 320, 420, 520), characterized in that the discharge hole (238, 338, 438, 538, 638, 639) includes a tangent direction of the inner circumference of the second housing hole at the connection position. a stir pin (220, 320, 420, 520) comprising a rotation axis; and a housing (210, 310, 410, 510) comprising a surface (234a, 334a, 434a, 534a, 634a), wherein the housing (210, 310, 410, 510) contains the stir pin (220, 320, 420, 520) rotatably about the rotation axis relative to the housing (210, 310, 410, 510), wherein the housing (210, 310, 410, 510) comprises a housing hole opening on the surface (234a, 334a, 434a, 534a, 634a), wherein the stir pin (220, 320, 420, 520) comprises a stir portion that is located at one end of the rotation axis, that is contained in the housing hole with a part of the stir portion protruding through and beyond the surface (234a, 334a, 434a, 534a, 634a), and that is to be inserted into a workpiece, wherein the housing (210, 310, 410, 510) comprises a discharge hole (238, 338, 438, 538, 638, 639) through which the housing hole is connected to an outside of the housing (210, 310, 410, 510), wherein the housing hole comprises a first housing hole and a second housing hole that are connected to each other along the rotation axis, wherein the first housing hole is open on the surface (234a, 334a, 434a, 534a, 634a), wherein the second housing hole has a hole diameter larger than a hole diameter of the first housing hole, wherein the discharge hole (238, 338, 438, 538, 638, 639) is located at a position including a boundary between the first housing hole and the second housing hole, wherein the housing (210, 310, 410, 510) comprises a shoulder (230, 330, 430, 530, 630) and a housing body (212, 312, 412, 512) that are separate from each other, the shoulder (230, 330, 430, 530, 630) comprising the surface (234a, 334a, 434a, 534a, 634a) and the discharge hole (238, 338, 438, 538, 638, 639), wherein the shoulder (230, 330, 430, 530, 630) comprises an attached portion (232, 332, 432, 632) that is attached to the housing body (212, 312, 412, 512), wherein the housing body (212, 312, 412, 512) comprises a holding portion (217, 317, 417) that holds the attached portion (232, 332, 432, 632), and wherein the shoulder (230, 330, 430, 530, 630) includes a connection position where an inner surface of the discharge hole (238, 338, 438, 538, 638, 639) and an inner surface of the second housing hole are connected to each other in the rotation direction of the stir pin (220, 320, 420, 520), characterized in that the discharge hole (238, 338, 438, 538, 638, 639) includes a tangent direction of the inner circumference of the second housing hole at the connection position.