Friction stir welding method and its device

A friction stir welding device for welding portions to be jointed of a work piece having a corner with a smaller curvature that connect two portions with larger curvatures, the device comprising:

• a supporting member that supports the work piece;
• a horizontally moving mechanism capable of moving the work piece on a horizontal surface;
• a vertically moving mechanism capable of vertically moving tool so as to penetrate in and depart from the portions to be joined;
• a direction changing mechanism capable of changing an advance angle direction of the tool whose center axis is inclined relative to an upper surface of the portions to be joined;
• a rotating mechanism capable of rotating the tool around the center axis of the tool;
• and
• a controller capable of controlling the horizontally moving mechanism, the vertically moving mechanism, the direction changing mechanism, and the rotating mechanism in such a way that a rotation speed of the tool around the center axis of the tool is decreased to be smaller at the corner with the smaller curvature than the rotation speed of the tool around the center axis of the tool at the portions with the larger curvatures.