A friction stir welding method in which a rotary tool and a backing tool are arranged on front and back surface sides of a joint portion of two butted metal plates (1, 2), the rotary tool frictionally stirs the joint portion and performs friction stir welding on the two metal plates, wherein: the rotary tool includes a tool body having a shoulder portion formed at a tip end portion thereof and at least one protruding portion formed to protrude from the tip end portion of the tool body, wherein the backing tool includes a backing tool body having a pressing force maintaining surface portion formed at a tip end portion thereof and at least one recessed portion which is a cylindrical hole for receiving a tip end of the protruding portion when the two metal plates (1, 2) are to be welded, the method comprising the steps of: arranging the rotary tool and the backing tool in opposed relationship on the front and back surface sides of the joint portion of the two metal plates; rotating the rotary tool; moving the rotary tool in such a direction that the rotary tool approaches the backing tool; inserting the tip end of the protruding portion of the rotary tool into the recessed portion of the backing tool; sandwiching the joint portion between a shoulder surface of the shoulder portion of the rotary tool and a pressing force maintaining surface of the backing tool; pressing the shoulder surface of the shoulder portion of the rotary tool and the pressing force maintaining surface of the backing tool against the front and back surfaces of the joint portion, respectively; moving the rotary tool and the backing tool along the joint portion in synchronization with each other; and frictionally stirring the joint portion in the entire region in the thickness direction of the metal plates by use of the rotary tool. the rotary tool includes a tool body having a shoulder portion formed at a tip end portion thereof and at least one protruding portion formed to protrude from the tip end portion of the tool body, wherein the backing tool includes a backing tool body having a pressing force maintaining surface portion formed at a tip end portion thereof and at least one recessed portion which is a cylindrical hole for receiving a tip end of the protruding portion when the two metal plates (1, 2) are to be welded, the method comprising the steps of: arranging the rotary tool and the backing tool in opposed relationship on the front and back surface sides of the joint portion of the two metal plates; rotating the rotary tool; moving the rotary tool in such a direction that the rotary tool approaches the backing tool; inserting the tip end of the protruding portion of the rotary tool into the recessed portion of the backing tool; sandwiching the joint portion between a shoulder surface of the shoulder portion of the rotary tool and a pressing force maintaining surface of the backing tool; pressing the shoulder surface of the shoulder portion of the rotary tool and the pressing force maintaining surface of the backing tool against the front and back surfaces of the joint portion, respectively; moving the rotary tool and the backing tool along the joint portion in synchronization with each other; and frictionally stirring the joint portion in the entire region in the thickness direction of the metal plates by use of the rotary tool. arranging the rotary tool and the backing tool in opposed relationship on the front and back surface sides of the joint portion of the two metal plates; rotating the rotary tool; moving the rotary tool in such a direction that the rotary tool approaches the backing tool; inserting the tip end of the protruding portion of the rotary tool into the recessed portion of the backing tool; sandwiching the joint portion between a shoulder surface of the shoulder portion of the rotary tool and a pressing force maintaining surface of the backing tool; pressing the shoulder surface of the shoulder portion of the rotary tool and the pressing force maintaining surface of the backing tool against the front and back surfaces of the joint portion, respectively; moving the rotary tool and the backing tool along the joint portion in synchronization with each other; and frictionally stirring the joint portion in the entire region in the thickness direction of the metal plates by use of the rotary tool.