A friction stir welding device comprising: a welding tool which rotates while abutting on a member to be welded to cause a plastic flow phenomenon in the member to be welded; a tool holder which grasps the welding tool and rotates together with the welding tool; supply paths (202, 203, 204, 252) which are formed in the tool holder and supply a refrigerant to the welding tool; collecting paths (213, 214, 253, 254) which are formed in the tool holder and collect the refrigerant from the welding tool; a cooling unit which cools down the refrigerant collected from the collecting paths and returns the refrigerant to the supply paths; a support portion which supports the tool holder for it to be freely rotated; a temperature sensor which is embedded in the welding tool; a control unit, wherein a first flow path and a second flow path are formed between the tool holder and the support portion facing each other, the supply paths (202, 203, 204, 252) communicate with the first flow path through which an outgoing refrigerant flows, the collecting paths (213, 214, 253, 254) communicate with the second flow path through which an incoming refrigerant flows, and the cooling unit includes a refrigerant tank which stores the refrigerant, a pump which circulates the refrigerant, and a cooler which cools down the refrigerant inside the refrigerant tank, supplies the refrigerant to the first flow path, and collects the refrigerant from the second flow path, and the control unit is configured to control the discharge speed by the pump based on a deviation between a tool temperature detected by the temperature sensor and a target temperature of the working tool, characterized in that the first flow path is a first annular flow path and the second flow path is a second annular flow path, wherein the first and second annular flow paths include respectively a support-portion-side concave portion (306, 316) which is a ring-shaped groove formed to go around an inner peripheral surface of the support portion along a circumference direction, and a tool-holder-side concave portion (206, 216) which is a ring-shaped groove formed in the tool holder to face the support-portion-side concave portion. a welding tool which rotates while abutting on a member to be welded to cause a plastic flow phenomenon in the member to be welded; a tool holder which grasps the welding tool and rotates together with the welding tool; supply paths (202, 203, 204, 252) which are formed in the tool holder and supply a refrigerant to the welding tool; collecting paths (213, 214, 253, 254) which are formed in the tool holder and collect the refrigerant from the welding tool; a cooling unit which cools down the refrigerant collected from the collecting paths and returns the refrigerant to the supply paths; a support portion which supports the tool holder for it to be freely rotated; a temperature sensor which is embedded in the welding tool; a control unit, wherein a first flow path and a second flow path are formed between the tool holder and the support portion facing each other, the supply paths (202, 203, 204, 252) communicate with the first flow path through which an outgoing refrigerant flows, the collecting paths (213, 214, 253, 254) communicate with the second flow path through which an incoming refrigerant flows, and the cooling unit includes a refrigerant tank which stores the refrigerant, a pump which circulates the refrigerant, and a cooler which cools down the refrigerant inside the refrigerant tank, supplies the refrigerant to the first flow path, and collects the refrigerant from the second flow path, and the control unit is configured to control the discharge speed by the pump based on a deviation between a tool temperature detected by the temperature sensor and a target temperature of the working tool, characterized in that the first flow path is a first annular flow path and the second flow path is a second annular flow path, wherein the first and second annular flow paths include respectively a support-portion-side concave portion (306, 316) which is a ring-shaped groove formed to go around an inner peripheral surface of the support portion along a circumference direction, and a tool-holder-side concave portion (206, 216) which is a ring-shaped groove formed in the tool holder to face the support-portion-side concave portion.