A double-acting friction stir spot welding device (10A, 10B) for welding a workpiece by partially stirring the workpiece with rotary tools (11, 12) comprising a holding jig configured to hold the workpiece, the friction stir spot welding device including, as the rotary tools, a cylindrical pin member that is configured to rotate around an axis and be movable forward and backward along the axis, a cylindrical shoulder member that is positioned so as to surround an outside of the pin member and is configured to rotate around the axis of the pin member and be movable forward and backward along the axis, and a tool driving unit that is configured to cause the rotary tools to rotate around the axis and move forward and backward along the axis, wherein the holding jig is a clamp member that has a cylindrical shape positioned so as to surround an outside of the shoulder member and is configured to press the workpiece from an obverse surface with an annular pressing surface of a distal end of the clamp member, the tool driving unit is adapted to partially stir the workpiece with the rotary tools (11, 12), the tool driving unit is adapted to make the shoulder member protrude from the pin member to press-fit the shoulder member into the workpiece, characterized in that the clamp member having an inclined surface that is adjacent to an inner edge portion of the pressing surface and inclined so as to reduce an inner diameter of the clamp member toward a back side as viewed from the pressing surface, the tool driving unit is adapted to cause the shoulder member to move to align a contact surface of the shoulder member with an upper edge of the inclined surface of the clamp member, such that a concave region (S1, S2) is formed by the inclined surface of the clamp member and the contact surface of the shoulder member, and the concave region is configured to be filled with a material constituting the workpiece, the material being softened by heat generated by friction of the rotary tools (11, 12). the friction stir spot welding device including, as the rotary tools, a cylindrical pin member that is configured to rotate around an axis and be movable forward and backward along the axis, a cylindrical shoulder member that is positioned so as to surround an outside of the pin member and is configured to rotate around the axis of the pin member and be movable forward and backward along the axis, and a tool driving unit that is configured to cause the rotary tools to rotate around the axis and move forward and backward along the axis, wherein the holding jig is a clamp member that has a cylindrical shape positioned so as to surround an outside of the shoulder member and is configured to press the workpiece from an obverse surface with an annular pressing surface of a distal end of the clamp member, the tool driving unit is adapted to partially stir the workpiece with the rotary tools (11, 12), the tool driving unit is adapted to make the shoulder member protrude from the pin member to press-fit the shoulder member into the workpiece, characterized in that the clamp member having an inclined surface that is adjacent to an inner edge portion of the pressing surface and inclined so as to reduce an inner diameter of the clamp member toward a back side as viewed from the pressing surface, the tool driving unit is adapted to cause the shoulder member to move to align a contact surface of the shoulder member with an upper edge of the inclined surface of the clamp member, such that a concave region (S1, S2) is formed by the inclined surface of the clamp member and the contact surface of the shoulder member, and the concave region is configured to be filled with a material constituting the workpiece, the material being softened by heat generated by friction of the rotary tools (11, 12).