A friction stir spot joining device for softening and stirring an object to be joined by frictional heat generated by rotation of a pin so as to perform spot joining, said friction stir spot joining device comprising: - a fixed arm; - a friction stir spot joining device body connected to the fixed arm and incorporating therein a stirring shaft supported by bearings and having the pin at the tip end and a hole at the center thereof through which a screw shaft can be housed from the rear defined as opposed to the pin, a stirring motor for rotating the stirring shaft, a power transmission mechanism disposed between the stirring shaft and the stirring motor, and provided with rails of a linear motion guide provided at a plate thereof at the bottom, that is the side opposite to the connection between the spot joining device body and the fixed arm; and - a pressure application motor fixed to a rear end of the friction stir spot joining device body; - the stirring motor, the stirring shaft and the power transmission mechanism being unitized to form a stirring unit; an upwardly extended support member for supporting the stirring motor is fixed to the rear end portion of a driving unit body that holds outer rings of the bearings of the stirring shaft, - bearings (19, 19) of the linear motion guide fixedly secured to the stirring unit to the lower portion of the driving unit body immediately under the driving unit body and a ball nut fixed to the back face of the stirring unit, and wherein the screw shaft connected to the pressure application motor is caused to screw with the ball nut fixed to the back face of the stirring unit so as to advance or retract the stirring unit by the pressure application motor; characterized in that the friction stir spot joining device body is opened to form an opening at a rear upper portion through which the stirring unit is caused to move inside thereof so that the stirring motor is exposed outside through the opening and rails 21 for guiding the movement of the bearings (19, 19) of the linear motor guide are provided o the plate of the joining device body at the bottom for a length exceeding a moving stroke of the bearings (19, 19) of the linear motor guide when performing the friction spot joining, so that the stirring motor and the power transmission mechanism can be easily taken at from the opening. - a fixed arm; - a friction stir spot joining device body connected to the fixed arm and incorporating therein a stirring shaft supported by bearings and having the pin at the tip end and a hole at the center thereof through which a screw shaft can be housed from the rear defined as opposed to the pin, a stirring motor for rotating the stirring shaft, a power transmission mechanism disposed between the stirring shaft and the stirring motor, and provided with rails of a linear motion guide provided at a plate thereof at the bottom, that is the side opposite to the connection between the spot joining device body and the fixed arm; and - a pressure application motor fixed to a rear end of the friction stir spot joining device body; - the stirring motor, the stirring shaft and the power transmission mechanism being unitized to form a stirring unit; an upwardly extended support member for supporting the stirring motor is fixed to the rear end portion of a driving unit body that holds outer rings of the bearings of the stirring shaft, - bearings (19, 19) of the linear motion guide fixedly secured to the stirring unit to the lower portion of the driving unit body immediately under the driving unit body and a ball nut fixed to the back face of the stirring unit, and wherein the screw shaft connected to the pressure application motor is caused to screw with the ball nut fixed to the back face of the stirring unit so as to advance or retract the stirring unit by the pressure application motor;