A friction stir welding apparatus adapted for welding a processing target member (W, W1, W2), comprising: a mounting member on which the processing target member (W, W1, W2) is to be mounted; a welding tool having a probe that is movable in a vertical direction defined by the z-axis with respect to a processing-target side surface (Ws) of the processing target member (W, W1, W2) and is rotatable relative to the processing target member (W, W1, W2) around a central axis Z, which is parallel to the z-axis, when the processing target member (W, W1, W2) is mounted to the mounting member; a movement mechanism having an arm fitted with the welding tool that can move the welding tool with respect to the processing target member (W, W1, W2) by moving the arm; and a correction mechanism having a correction member that has a contact surface parallel to a predetermined processing direction on the processing-target side surface (Ws) and that can correct a moving direction of the welding tool so as to be matched with the predetermined processing direction, and being fixed to the mounting member, characterized in that - the welding tool has an interposed member that covers a part of an outer periphery of the probe, wherein the interposed member is a bearing rotatable relative to the probe coaxially with the central axis Z, and further characterized by - a controller, comprising an arithmetic processing unit and a memory, wherein the control program for performing friction stir welding and the predetermined processing direction are stored in the memory, the controller being adapted to control the movement mechanism, the welding tool and the correction mechanism such that - the correction member is arranged on a left or right side with respect to the processing direction, when a rotation direction of the probe is clockwise or counterclockwise with respect to the processing-target side surface (Ws), respectively, and - the correction member is adapted to correct the moving direction of the welding tool by bringing the interposed member into contact with the contact surface of the correction member when the welding tool is moved with respect to the processing target member (W, W1, W2) by moving the arm. a mounting member on which the processing target member (W, W1, W2) is to be mounted; a welding tool having a probe that is movable in a vertical direction defined by the z-axis with respect to a processing-target side surface (Ws) of the processing target member (W, W1, W2) and is rotatable relative to the processing target member (W, W1, W2) around a central axis Z, which is parallel to the z-axis, when the processing target member (W, W1, W2) is mounted to the mounting member; a movement mechanism having an arm fitted with the welding tool that can move the welding tool with respect to the processing target member (W, W1, W2) by moving the arm; and a correction mechanism having a correction member that has a contact surface parallel to a predetermined processing direction on the processing-target side surface (Ws) and that can correct a moving direction of the welding tool so as to be matched with the predetermined processing direction, and being fixed to the mounting member, characterized in that - the welding tool has an interposed member that covers a part of an outer periphery of the probe, wherein the interposed member is a bearing rotatable relative to the probe coaxially with the central axis Z, and further characterized by - a controller, comprising an arithmetic processing unit and a memory, wherein the control program for performing friction stir welding and the predetermined processing direction are stored in the memory, the controller being adapted to control the movement mechanism, the welding tool and the correction mechanism such that - the correction member is arranged on a left or right side with respect to the processing direction, when a rotation direction of the probe is clockwise or counterclockwise with respect to the processing-target side surface (Ws), respectively, and - the correction member is adapted to correct the moving direction of the welding tool by bringing the interposed member into contact with the contact surface of the correction member when the welding tool is moved with respect to the processing target member (W, W1, W2) by moving the arm. - the welding tool has an interposed member that covers a part of an outer periphery of the probe, wherein the interposed member is a bearing rotatable relative to the probe coaxially with the central axis Z, and further characterized by - a controller, comprising an arithmetic processing unit and a memory, wherein the control program for performing friction stir welding and the predetermined processing direction are stored in the memory, the controller being adapted to control the movement mechanism, the welding tool and the correction mechanism such that - the correction member is arranged on a left or right side with respect to the processing direction, when a rotation direction of the probe is clockwise or counterclockwise with respect to the processing-target side surface (Ws), respectively, and - the correction member is adapted to correct the moving direction of the welding tool by bringing the interposed member into contact with the contact surface of the correction member when the welding tool is moved with respect to the processing target member (W, W1, W2) by moving the arm.