The friction stir spot welding tool (1), by the pressure from the first member side to overlap the second member and the first member, the bonding is to first member and the second member a friction stir spot. The first and second members are, respectively, a tensile strength of more than 270 370 ㎫ ㎫ than steel. The friction stir spot welding tool (1) is rotatable about an axis of rotation (4). The friction stir spot welding tool (1), the front end on the shoulder (5) of the pillar (2), a shoulder (5), the rotating shaft 4 protruding cylinder in extending direction-shaped or truncated cone-shaped from having a probe and a (3). The when the volume to be inserted into the second member side to the second member insertion volume V (㎣) when the thickness of the second member to the t (㎜), and the probe (3) is pressure, V is in the relationship between t equation (1) V≥1.73t 2 + 5.12t-3.56 (t≥0.5 ㎜) ... (One) Satisfies.
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