The present invention provides a friction stir welder that includes a stage on which at least two material pieces to be welded are mounted and stacked, a pressing member that covers and presses end faces of the material pieces to be welded that are mounted and stacked on the stage, and a rotary tool that is inserted into the stacked material pieces to be welded while rotating, so that a plastic flow is generated at an interfacing portion as well as in the vicinity thereof between the rotary tool and the material pieces to be welded, thereby to joint the material pieces to be welded to each other.