There is provided a friction stir spot joining device wherein stirring-related members are unitized to form a unitized stirring unit and an opening is formed in a friction stir spot joining device body for housing the unitized stirring unit therein. A stirring motor, a stirring shaft and a power transmission mechanism are unitized to form a stirring unit, and bearings of a linear motion guide are fixed to the stirring unit immediately under the stirring unit, and a ball nut is fixed to the back face of the stirring unit, wherein the friction stir spot joining device body is opened to form the opening at a rear upper portion through which the stirring unit is caused to move inside thereof.