A friction stir welding apparatus has a probe rotatable and vertically movable with respect to a processing target member, a welding tool having an interposed member that covers a part of an outer periphery of the probe and is rotatable relative to the probe, a mounting member on which the processing target member is mounted, a movement mechanism having an arm fitted with the welding tool that can move the welding tool with respect to the processing target member by moving the arm, and a correction mechanism having a correction member that can correct a moving direction of the welding tool so as to be matched with a predetermined processing direction, by bringing the interposed member into contact with the processing target member when the welding tool is moved with respect to the processing target member by moving the arm, and fixed to the mounting member.