The purpose of the present invention is to provide a lightweight and compact friction stir welding device equipped with an upper rotating body and a lower rotating body. This friction stir welding device (10) is configured so as to be equipped with: an upper rotating body (36); a lower rotating body (15); a rotating principal shaft (14) for passing through the interior of the upper rotating body and integrally provided with the lower rotating body; a first actuator (16) for imparting a rotational force on the rotating principal shaft; a second actuator (27) for moving the upper rotating body in the axial direction; a load sensor (39) for detecting an axial load applied to the second actuator; a load control unit (41) for controlling the load to be applied to a workpiece by the upper rotating body, on the basis of load information obtained from the load sensor; a position sensor (43) for detecting the position of the lower rotating body; and a controller (44) for controlling the position of the lower rotating body on the basis of the position information from the position sensor.
