Providing a rotary tool for friction stir spot-welding, which makes it possible to join together two aluminum members superposed on each other, with a high degree of joint strength. The rotary tool has a probe which is provided in a central part of a distal end portion of a cylindrical tool body so as to extend coaxially with the tool body and which has a stepped structure consisting of an intermediate probe and a central probe which extend coaxially with each other. Distal end faces of the tool body and the intermediate probe respectively give a first shoulder surface and a second shoulder surface which are gradually recessed in a radially inward direction and connected to proximal portions of the respective intermediate and central probes.