A friction stir spot welder adapted to weld workpieces through softening induced by friction heat, the workpieces including first and second workpieces stacked in the presence of a sealant applied to a region of contact between the first and second workpieces, the friction stir spot welder comprising: a pin in the form of a solid circular cylinder; a shoulder in the form of a hollow circular cylinder, the shoulder having an interior in which the pin is inserted; a rotary actuator that rotates the pin and the shoulder about an axis coinciding with a central axis of the pin; a clamp in the form of a hollow circular cylinder, the clamp having an interior in which the pin and the shoulder are inserted; an advancement/withdrawal actuator that advances and withdraws the pin, the shoulder, and the clamp along the axis; and a controller, and characterized in that the controller is bung configured to: (B) drive the rotary actuator and the advancement/withdrawal actuator to cause the pin to press the workpieces while rotating; (C) after performing the pressing (B) for a first predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and the shoulder to press the workpieces while rotating; and (D) after performing the pressing (C) for a second predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and/or the shoulder to plunge into a weld region of the workpieces while rotating and stir the weld region to weld the workpieces together. a pin in the form of a solid circular cylinder; a shoulder in the form of a hollow circular cylinder, the shoulder having an interior in which the pin is inserted; a rotary actuator that rotates the pin and the shoulder about an axis coinciding with a central axis of the pin; a clamp in the form of a hollow circular cylinder, the clamp having an interior in which the pin and the shoulder are inserted; an advancement/withdrawal actuator that advances and withdraws the pin, the shoulder, and the clamp along the axis; and a controller, and characterized in that the controller is bung configured to: (B) drive the rotary actuator and the advancement/withdrawal actuator to cause the pin to press the workpieces while rotating; (C) after performing the pressing (B) for a first predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and the shoulder to press the workpieces while rotating; and (D) after performing the pressing (C) for a second predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and/or the shoulder to plunge into a weld region of the workpieces while rotating and stir the weld region to weld the workpieces together. (B) drive the rotary actuator and the advancement/withdrawal actuator to cause the pin to press the workpieces while rotating; (C) after performing the pressing (B) for a first predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and the shoulder to press the workpieces while rotating; and (D) after performing the pressing (C) for a second predetermined time, drive the rotary actuator and the advancement/withdrawal actuator to cause the pin and/or the shoulder to plunge into a weld region of the workpieces while rotating and stir the weld region to weld the workpieces together. <EOS>