A friction stir welding apparatus having a new cooling system that can cool a portion required to be cooled accurately and uniformly with a simple configuration, while suppressing unnecessary thermal expansion of a mounting jig includes a holding mechanism 20 having a holding member 22a movable with respect to a processing target member W so as to freely hold the processing target member W, in which the holding member 22a and a portion of a mounting jig 12 that can freely face the holding member 22a become paired, and a coolant supply source S that can freely supply a coolant to the paired holding member 22a and the portion of the mounting jig 12.