A movable trailing edge is composed of a main body including a structural frame member in which a spar and a rib intersect with each other, and a first outer plate that covers the main body. The first outer plate is placed on and cover the structural frame member, a rotating probe is pressed against the first outer plate from the obverse side thereof to friction stir weld the first outer plate and the side end face of the structural frame member. Joint parts are arranged in a dotted pattern.