Provided are a friction stir welding device and a friction stir welding method that make it possible: to control, in accordance with welding conditions and the state of members to be welded, the temperature difference between an advancing side (AS) and a retreating side (RS) during friction stir welding; and to achieve high-quality (high-precision) welding. A friction stir welding device that comprises: a welding tool that is configured from a shoulder part and a probe part and that is inserted between members to be welded and rotates; a device main body that holds the welding tool; a control device that controls the operation of the welding tool; a first temperature sensor that measures the temperature of an advancing side (AS) of the members to be welded; and a second temperature sensor that measures the temperature of a retreating side (RS) of the members to be welded. The friction stir welding device is characterized in that the first temperature sensor and the second temperature sensor: are arranged on the device main body to measure welding temperatures near the circumference of the shoulder part at locations that are left-right symmetric with respect to a line along which welding occurs when the welding tool advances; and are also arranged to measure the welding temperatures while avoiding flash generated at the surfaces of the members to be welded when the members to be welded are friction stir welded. The friction stir welding device is also characterized in that, on the basis of the welding temperatures measured by the first temperature sensor and the second temperature sensor, the friction stir welding device controls the rotational speed and/or the advancement speed of the welding tool.
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