The abutment portion touches with a shoulder surface of a joiningtool for friction stir welding. The tip portion is freely movable in a first direction connecting a surface of the workpiece and the shoulder surface in a state of contacting the surface of the workpiece. The detector is configured to detect a position of the tip portion in the first direction. The output unit is configured to output a data of the position of the tip portion in the first direction detected by the detector. The controller is configured to calculate a position of the tip portion to the abutment portion from the data of the position output from the output unit, and to calculate a distance between the shoulder surface of the joiningtool and the surface of the workpiece.
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