A friction stir spot welding apparatus including a rotary tool configured to move forward and backward along a rotational axis, the friction stir spot welding apparatus being suitable for pressing a distal end of the rotary tool on workpieces , the workpieces being a first member and a second member , which are laid one on top of the other and between which a sealant is applied to a contacting portion of the first member and a contacting portion of the second member , the contacting portions contacting each other; and for rotating a contacting part of the rotary tool, the contacting part contacting the workpieces , such that the workpieces are softened by frictional heat and stirred to be joined together, the friction stir spot welding apparatus comprising: a columnar pin member ; a cylindrical shoulder member , in which the pin member is inserted; a rotation driver configured to rotate the pin member and the shoulder member about an axis that coincides with a central axis of the pin member ; a cylindrical clamp member , in which the pin member and the shoulder member are inserted; a forward/backward movement driver configured to move each of the pin member , the shoulder member , and the clamp member forward and backward along the axis; and a controller , wherein the friction stir spot welding apparatus is characterized in that the controller is configured to perform a preliminary operation of controlling the forward/backward movement driver , such that at least one of the pin member and the shoulder member presses the workpieces before the clamp member presses the workpieces ; and then to perform a joining operation of controlling the rotation driver and the forward/backward movement driver , such that the pin member and the shoulder member stir the workpieces . a columnar pin member ; a cylindrical shoulder member , in which the pin member is inserted; a rotation driver configured to rotate the pin member and the shoulder member about an axis that coincides with a central axis of the pin member ; a cylindrical clamp member , in which the pin member and the shoulder member are inserted; a forward/backward movement driver configured to move each of the pin member , the shoulder member , and the clamp member forward and backward along the axis; and a controller , wherein the friction stir spot welding apparatus is characterized in that the controller is configured to perform a preliminary operation of controlling the forward/backward movement driver , such that at least one of the pin member and the shoulder member presses the workpieces before the clamp member presses the workpieces ; and then to perform a joining operation of controlling the rotation driver and the forward/backward movement driver , such that the pin member and the shoulder member stir the workpieces . perform a preliminary operation of controlling the forward/backward movement driver , such that at least one of the pin member and the shoulder member presses the workpieces before the clamp member presses the workpieces ; and then to perform a joining operation of controlling the rotation driver and the forward/backward movement driver , such that the pin member and the shoulder member stir the workpieces . <EOS>