A friction stir welding device comprising: a rotary tool having a cylindrical shoulder part rotatable about an axis of rotation and a probe coupled to a lower end of the shoulder part, rotated about a same axis as the axis of rotation, and protruding downward so as to contact members to be welded; a tool holder connected to an upper side of the rotary tool and rotated about a same axis as the rotary tool in cooperation with the rotary tool; and a device body having a mechanism connected to an upper side of the tool holder, rotating the rotary tool and the tool holder about the axis of rotation, and moving the rotary tool and the tool holder in an axial direction so as to press the members to be welded, characterized in that the rotary tool at least comprises: a hollow lower end channel extending from an upper end of the shoulder part to a vicinity of a lower end of the probe; and a hollow outer edge channel spaced apart from the axis of rotation in a radial direction to a vicinity of an outer edge of the shoulder part and extending from the upper end of the shoulder part to a vicinity of a lower end of the shoulder part, wherein temperature measurement elements are provided in the lower end channel and the outer edge channel, the temperature measurement elements being located in a vicinity of lower ends of the lower end channel and the outer edge channel, and wherein the friction stir welding device further comprises a transmission means for transmitting a temperature measurement result created using a temperature measurement means from the respective temperature measurement elements. a rotary tool having a cylindrical shoulder part rotatable about an axis of rotation and a probe coupled to a lower end of the shoulder part, rotated about a same axis as the axis of rotation, and protruding downward so as to contact members to be welded; a tool holder connected to an upper side of the rotary tool and rotated about a same axis as the rotary tool in cooperation with the rotary tool; and a device body having a mechanism connected to an upper side of the tool holder, rotating the rotary tool and the tool holder about the axis of rotation, and moving the rotary tool and the tool holder in an axial direction so as to press the members to be welded, characterized in that the rotary tool at least comprises: a hollow lower end channel extending from an upper end of the shoulder part to a vicinity of a lower end of the probe; and a hollow outer edge channel spaced apart from the axis of rotation in a radial direction to a vicinity of an outer edge of the shoulder part and extending from the upper end of the shoulder part to a vicinity of a lower end of the shoulder part, wherein temperature measurement elements are provided in the lower end channel and the outer edge channel, the temperature measurement elements being located in a vicinity of lower ends of the lower end channel and the outer edge channel, and wherein the friction stir welding device further comprises a transmission means for transmitting a temperature measurement result created using a temperature measurement means from the respective temperature measurement elements. a hollow lower end channel extending from an upper end of the shoulder part to a vicinity of a lower end of the probe; and a hollow outer edge channel spaced apart from the axis of rotation in a radial direction to a vicinity of an outer edge of the shoulder part and extending from the upper end of the shoulder part to a vicinity of a lower end of the shoulder part, wherein temperature measurement elements are provided in the lower end channel and the outer edge channel, the temperature measurement elements being located in a vicinity of lower ends of the lower end channel and the outer edge channel, and wherein the friction stir welding device further comprises a transmission means for transmitting a temperature measurement result created using a temperature measurement means from the respective temperature measurement elements.