A method for friction stir welding to join two metal members (31, 32) with use of a rotary tool equipped with a stirring pin, comprising: an overlapped portion forming process to overlap a front surface of one metal member and a back surface of the other metal member to form an overlapped portion; a primary joining process to insert the stirring pin, which is rotating, from a front surface of the other metal member, and to perform friction stirring to the overlapped portion in a state of only the stirring pin being in contact with both of the one metal member and the other metal member or with only the other metal member while a base part of the stirring pin is exposed; and a deburring process to remove burrs generated by friction stirring using the rotary tool after finishing the primary joining process; wherein the stirring pin has a continuously tapered shape so as to have a smaller diameter in a hanging down direction and the stirring pin has a spiral groove formed on an outer peripheral surface, and wherein the spiral groove is formed so as to rotate in a counter-clockwise direction as it goes downward in a case where the rotary tool is rotated in the clockwise direction, and the spiral groove is formed so as to rotate in the clockwise direction as it goes downward in a case where the rotary tool is rotated in the counter-clockwise direction. an overlapped portion forming process to overlap a front surface of one metal member and a back surface of the other metal member to form an overlapped portion; a primary joining process to insert the stirring pin, which is rotating, from a front surface of the other metal member, and to perform friction stirring to the overlapped portion in a state of only the stirring pin being in contact with both of the one metal member and the other metal member or with only the other metal member while a base part of the stirring pin is exposed; and a deburring process to remove burrs generated by friction stirring using the rotary tool after finishing the primary joining process; wherein the stirring pin has a continuously tapered shape so as to have a smaller diameter in a hanging down direction and the stirring pin has a spiral groove formed on an outer peripheral surface, and wherein the spiral groove is formed so as to rotate in a counter-clockwise direction as it goes downward in a case where the rotary tool is rotated in the clockwise direction, and the spiral groove is formed so as to rotate in the clockwise direction as it goes downward in a case where the rotary tool is rotated in the counter-clockwise direction. a deburring process to remove burrs generated by friction stirring using the rotary tool after finishing the primary joining process; wherein the stirring pin has a continuously tapered shape so as to have a smaller diameter in a hanging down direction and the stirring pin has a spiral groove formed on an outer peripheral surface, and wherein the spiral groove is formed so as to rotate in a counter-clockwise direction as it goes downward in a case where the rotary tool is rotated in the clockwise direction, and the spiral groove is formed so as to rotate in the clockwise direction as it goes downward in a case where the rotary tool is rotated in the counter-clockwise direction.