A both-side friction stir welding method in which first and second rotary tools (3, 4) are arranged in opposed relationship in the front and back surface sides of a joint portion of two metal plates (1, 2) to be joined, and the joint portion is frictionally stirred by the rotary tools (3, 4) to perform friction stir welding and join the two metal plates, wherein: one of the first and second rotary tools (3, 4) 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, characterized in that the other of the first and second rotary tools (3, 4) includes a tool body having a shoulder portion formed at the tip end portion thereof and at least one recessed portion formed at the tip end portion of the other tool body, wherein the recessed portion is formed to receive the tip end of the protruding portion therein when the two metal plates (1, 2) are to be welded; the method being further characterized by the steps of: arranging the first and second rotary tools in opposed relationship in the front and back surface sides of the joint portion of the two metal plates; rotating the first and second rotary tools; and moving the first and second rotary tools in a direction to approach to each other so as to insert the tip end of the protruding portion of the first rotary tool into the recessed portion of the second rotary tool while pressing shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools against the front and back surfaces of the joint portion; and moving, under the state that the tip end of the protruding portion of the first rotary tool is inserted into the recessed portion of the second rotary tool and the shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools are pressed against the front and back surfaces of the joint portion, the first and second rotary tools along the joint portion thereby to frictionally stir the joint portion in the entire region in the thickness direction of the joint portion. one of the first and second rotary tools (3, 4) 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, characterized in that the other of the first and second rotary tools (3, 4) includes a tool body having a shoulder portion formed at the tip end portion thereof and at least one recessed portion formed at the tip end portion of the other tool body, wherein the recessed portion is formed to receive the tip end of the protruding portion therein when the two metal plates (1, 2) are to be welded; the method being further characterized by the steps of: arranging the first and second rotary tools in opposed relationship in the front and back surface sides of the joint portion of the two metal plates; rotating the first and second rotary tools; and moving the first and second rotary tools in a direction to approach to each other so as to insert the tip end of the protruding portion of the first rotary tool into the recessed portion of the second rotary tool while pressing shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools against the front and back surfaces of the joint portion; and moving, under the state that the tip end of the protruding portion of the first rotary tool is inserted into the recessed portion of the second rotary tool and the shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools are pressed against the front and back surfaces of the joint portion, the first and second rotary tools along the joint portion thereby to frictionally stir the joint portion in the entire region in the thickness direction of the joint portion. arranging the first and second rotary tools in opposed relationship in the front and back surface sides of the joint portion of the two metal plates; rotating the first and second rotary tools; and moving the first and second rotary tools in a direction to approach to each other so as to insert the tip end of the protruding portion of the first rotary tool into the recessed portion of the second rotary tool while pressing shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools against the front and back surfaces of the joint portion; and moving, under the state that the tip end of the protruding portion of the first rotary tool is inserted into the recessed portion of the second rotary tool and the shoulder surfaces (3b, 4b) of the shoulder portions of the first and second rotary tools are pressed against the front and back surfaces of the joint portion, the first and second rotary tools along the joint portion thereby to frictionally stir the joint portion in the entire region in the thickness direction of the joint portion.