Disclosed is a friction stir welding tool with an adjustable probe length and shoulder groove depth, wherein a support shaft, made of a material having low thermal conductivity so as to inhibit thermal conduction between a probe and a shank, is coupled to the center of an upper locking member together with the shank, the probe and inner and outer shoulders are coupled inside a lower part of the upper locking member, and a lower locking member is fastened and fixed to the lower part of the upper locking member, and a washer and a locking nut are fastened to an upper part of the lower locking member, whereby the locking nut pushes the lower locking member via the washer.