1. A friction stir welding device comprising:a joining head attached to a wrist head of a robot main body having multi-axis degrees of freedom;
a drive device having the robot main body and capable of moving the joining head along a joint line where two members to be joined are joined together;
a control device which supplies a motor drive current to an electric motor provided for the joining head, thus inserts a pin part of a joining tool into the members to be joined while rotating the joining tool, and also controls the drive device in such a way that the joining head moves along the joint line, thus performing friction stir welding of the two members to be joined; and
a deviation detection device which detects a junction deviation which is a deviation between the joining head that is moving and a direction along the joint line,
wherein the control device is configured to output to the drive device a position correction signal with a different polarity according to a difference in direction of misalignment between the joining head and the joint line, for moving the joining head in a direction toward the joint line and resolving the junction deviation, and thus moves the joining head toward the joint line, until a time the junction deviation becomes zero, said control device to output when the junction deviation exceeds a deviation limit value which is a predetermined maximum deviation at the time of the friction stir welding, and
wherein the control device is configured to stop outputting the position correction signal to the drive device until the next time the junction deviation exceeds the deviation limit value, said control device to stop outputting when the junction deviation becomes zero.