A friction stir spot welding apparatus including a friction stir spot welding unit comprising a rotary tool , a rotary tool driver , a backing portion , and a frame portion characterised in that the friction stir spot welding unit further comprises a perpendicular-to-plane detector , wherein the rotary tool moves forward and backward along a rotational axis, the apparatus pressing a tip of the rotary tool against workpieces and rotating a contacting portion of the tip, the contacting portion being in contact with the workpieces, to soften the workpieces by frictional heat such that the workpieces are stirred and joined together, wherein the perpendicular-to-plane detector detects whether or not the rotary tool is positioned to be in a perpendicular-to-plane state relative to a joining target portion of the workpieces, and being further characterised by : an arm unit configured to operate in a three-dimensional manner, and an arm control unit adapted to control the operation of the arm unit , wherein the friction stir spot welding unit is mounted to the arm unit , and the arm control unit is adapted to control the operation of the arm unit , such that all the distances to the measurement positions measured by the perpendicular-to-plane detector are equal to each other, wherein the perpendicular-to-plane detector includes position sensors that are disposed on a reference plane to which forward and backward movement directions of the rotary tool are normal, the position sensors measuring distances to at least three measurement positions that are set around the joining target portion, the perpendicular-to-plane detector is configured to detect, based on the distances to the measurement positions, that the rotary tool is positioned to be in the perpendicular-to-plane state relative to the joining target portion of the workpieces, each position sensor is adapted to use a measuring portion of the position sensor as a reference position, and to measure a distance from the reference position to a measurement position as a displacement, and the arm control unit is adapted to detect through comparison of these displacements that the rotary tool is positioned to be in the perpendicular-to-plane state relative to the joining target position of the workpieces. an arm unit configured to operate in a three-dimensional manner, and an arm control unit adapted to control the operation of the arm unit , wherein the friction stir spot welding unit is mounted to the arm unit , and the arm control unit is adapted to control the operation of the arm unit , such that all the distances to the measurement positions measured by the perpendicular-to-plane detector are equal to each other, wherein the perpendicular-to-plane detector includes position sensors that are disposed on a reference plane to which forward and backward movement directions of the rotary tool are normal, the position sensors measuring distances to at least three measurement positions that are set around the joining target portion, the perpendicular-to-plane detector is configured to detect, based on the distances to the measurement positions, that the rotary tool is positioned to be in the perpendicular-to-plane state relative to the joining target portion of the workpieces, each position sensor is adapted to use a measuring portion of the position sensor as a reference position, and to measure a distance from the reference position to a measurement position as a displacement, and the arm control unit is adapted to detect through comparison of these displacements that the rotary tool is positioned to be in the perpendicular-to-plane state relative to the joining target position of the workpieces. <EOS>