A friction stir welding tool comprising: a stir pin comprising a rotation axis; and a housing comprising a surface , wherein the housing contains the stir pin rotatably about the rotation axis relative to the housing , wherein the housing comprises a housing hole (SH) opening on the surface , wherein the stir pin comprises a stir portion that is located at one end of the rotation axis, that is contained in the housing hole (SH) with a part of the stir portion protruding through and beyond the surface , and that is to be inserted into a workpiece, wherein the housing comprises a discharge hole through which the housing hole (SH) is connected to an outside of the housing , wherein the housing hole (SH) comprises a first housing hole (H1) and a second housing hole (H2) that are connected to each other along the rotation axis, wherein the first housing hole (H1) is open on the surface , wherein the second housing hole (H2) has a hole diameter larger than a hole diameter of the first housing hole (H1), wherein the discharge hole is located at a position including a boundary (B) between the first housing hole (H1) and the second housing hole (H2), wherein the housing comprises a shoulder and a housing body that are separate from each other, the shoulder comprising the surface and the discharge hole , wherein the shoulder comprises an attached portion that is attached to the housing body , wherein the housing body comprises a holding portion that holds the attached portion , and wherein the shoulder includes a connection position (P1) where an inner surface of the discharge hole and an inner surface of the second housing hole (H2) are connected to each other in the rotation direction (R) of the stir pin , characterized in that the discharge hole includes a tangent direction (T1) of the inner circumference of the second housing hole (H2) at the connection position (P1). a stir pin comprising a rotation axis; and a housing comprising a surface , wherein the housing contains the stir pin rotatably about the rotation axis relative to the housing , wherein the housing comprises a housing hole (SH) opening on the surface , wherein the stir pin comprises a stir portion that is located at one end of the rotation axis, that is contained in the housing hole (SH) with a part of the stir portion protruding through and beyond the surface , and that is to be inserted into a workpiece, wherein the housing comprises a discharge hole through which the housing hole (SH) is connected to an outside of the housing , wherein the housing hole (SH) comprises a first housing hole (H1) and a second housing hole (H2) that are connected to each other along the rotation axis, wherein the first housing hole (H1) is open on the surface , wherein the second housing hole (H2) has a hole diameter larger than a hole diameter of the first housing hole (H1), wherein the discharge hole is located at a position including a boundary (B) between the first housing hole (H1) and the second housing hole (H2), wherein the housing comprises a shoulder and a housing body that are separate from each other, the shoulder comprising the surface and the discharge hole , wherein the shoulder comprises an attached portion that is attached to the housing body , wherein the housing body comprises a holding portion that holds the attached portion , and wherein the shoulder includes a connection position (P1) where an inner surface of the discharge hole and an inner surface of the second housing hole (H2) are connected to each other in the rotation direction (R) of the stir pin , characterized in that the discharge hole includes a tangent direction (T1) of the inner circumference of the second housing hole (H2) at the connection position (P1). <EOS>