A friction stir welding apparatus for performing friction stir welding on an inner corner portion that is formed by two metal members(91,92) butted together, the friction stir welding apparatus comprising: a rotary tool provided with a stirring pin at its distal end; a shoulder in which the rotary tool is inserted, the shoulder coming into contact with the two metal members(91,92); a rotation driving unit configured to cause the rotary tool to rotate about an axis; a pressing device configured to press the rotary tool and the shoulder against the inner corner portion; a moving device configured to move the rotary tool and the shoulder in an advancing direction along the inner corner portion, wherein the advancing direction is the direction extending along the butting line of the inner corner portion; characterized by a wire heating device configured to heat a wire that is friction-stirred together with the two metal members(91,92) by the stirring pin, and characterized in that the shoulder includes: an insertion hole in which the rotary tool is inserted; two shoulder surfaces(85,85) that are provided on both sides, respectively, of an opening of the insertion hole in a direction substantially perpendicular to the advancing direction, the two shoulder surfaces(85,85) forming an angle that corresponds to an angle, formed by the two metal members(91,92), of the inner corner portion and a guide groove intended for the wire, the guide groove extending parallel to the advancing direction and being formed in a front ridge portion where the two shoulder surfaces(85,85) meet, such that, in use, the front ridge portion is positioned forward from the opening of the insertion hole in the advancing direction. a rotary tool provided with a stirring pin at its distal end; a shoulder in which the rotary tool is inserted, the shoulder coming into contact with the two metal members(91,92); a rotation driving unit configured to cause the rotary tool to rotate about an axis; a pressing device configured to press the rotary tool and the shoulder against the inner corner portion; a moving device configured to move the rotary tool and the shoulder in an advancing direction along the inner corner portion, wherein the advancing direction is the direction extending along the butting line of the inner corner portion; characterized by a wire heating device configured to heat a wire that is friction-stirred together with the two metal members(91,92) by the stirring pin, and characterized in that the shoulder includes: an insertion hole in which the rotary tool is inserted; two shoulder surfaces(85,85) that are provided on both sides, respectively, of an opening of the insertion hole in a direction substantially perpendicular to the advancing direction, the two shoulder surfaces(85,85) forming an angle that corresponds to an angle, formed by the two metal members(91,92), of the inner corner portion and a guide groove intended for the wire, the guide groove extending parallel to the advancing direction and being formed in a front ridge portion where the two shoulder surfaces(85,85) meet, such that, in use, the front ridge portion is positioned forward from the opening of the insertion hole in the advancing direction. an insertion hole in which the rotary tool is inserted; two shoulder surfaces(85,85) that are provided on both sides, respectively, of an opening of the insertion hole in a direction substantially perpendicular to the advancing direction, the two shoulder surfaces(85,85) forming an angle that corresponds to an angle, formed by the two metal members(91,92), of the inner corner portion and a guide groove intended for the wire, the guide groove extending parallel to the advancing direction and being formed in a front ridge portion where the two shoulder surfaces(85,85) meet, such that, in use, the front ridge portion is positioned forward from the opening of the insertion hole in the advancing direction.