A friction stir spot-welding tool (1) for friction stir spot-welding a first member and a second member by superposing the first member and the second member and applying pressure from the first-member side. The first member and the second member are steel plates having a tensile strength of 270-370 MPa or greater. The friction stir spot-welding tool (1) is capable of rotating around an axis of rotation (4). The friction stir spot-welding tool (1) is equipped with: a columnar section (2) having at its tip end a shoulder (5); and a cylindrical or truncated conical probe (3) protruding from the shoulder (5) in the direction of elongation of the axis of rotation (4). When t (mm) is the thickness of the second member and the second member insertion volume V (mm3) is the volume of the insertion into the second member when the probe (3) is pressed from the first-member side, the relationship between V and t satisfies the equation (1). V ≥ 1.73t2 + 5.12t - 3.56 (t ≥ 0.5 mm) . . . . (1)
