Friction stir spot welding method and friction stir spot welding device

A device for performing friction stir spot welding of a plurality of steel plates, the device comprising:

• a displacement driving unit which displaces a spot welding portion of the plurality of steel plates and a tool relative to each other;
• a rotation driving unit which rotates the tool;
• and
• a control unit,
• wherein the control unit performs a friction stirring step, in which the control unit causes the displacement driving unit and the rotation driving unit to operate to press the tool against the spot welding portion while rotating the tool, to plasticize the spot welding portion by friction heat,
• wherein the control unit performs a cooling step for cooling the spot welding portion, in which the control unit causes at least one of the displacement driving unit and the rotation driving unit to operate, to pull up the tool and/or reduce a rotational speed of the tool, and
• wherein the control unit performs a tempering step for tempering the spot welding portion, in which the control unit causes the displacement driving unit and the rotation driving unit to operate to re-press the tool against the spot welding portion while rotating the tool.