A friction stir spot welding method for performing friction stir spot welding of a workpiece by using a rotary tool configured to partially stir the workpiece and a clamp member configured to press a surface of the workpiece while the workpiece is supported, the rotary tool including a pin member configured to rotate around a predetermined axis and advance and retract in an axial direction and a shoulder member configured to rotate around the axis while surrounding an outer periphery of the pin member and advance and retract in the axial direction independently of the pin member, characterized by the clamp member being a cylindrical member and including an inner peripheral surface that surrounds an outer periphery of the shoulder member to form an accommodation space for the shoulder member, an annular end face that comes into surface contact with the surface of the workpiece to press the surface, and a chamber portion interposed between the inner peripheral surface and the end face, recessed in a diameter-increasing direction with respect to the inner peripheral surface, and opening toward the workpiece, the friction stir spot welding method comprising: a welding step of forming a build-up portion on the workpiece by friction-stirring the workpiece by using the pin member and the shoulder member, while pressing the surface of the workpiece with the end face of the clamp member, and filling a portion of the workpiece being softened in the chamber portion by press-fitting at least one of the pin member and the shoulder member into a plastic flow portion of the workpiece; and a pressing step of pressing a surface of at least one of a region of the build-up portion which is frictionally stirred by the shoulder member and an adjacent region of the build-up portion which is adjacent to the friction-stirred region with the end face of the clamp member with the pin member and the shoulder member retracted into the accommodation space. the rotary tool including a pin member configured to rotate around a predetermined axis and advance and retract in an axial direction and a shoulder member configured to rotate around the axis while surrounding an outer periphery of the pin member and advance and retract in the axial direction independently of the pin member, characterized by the clamp member being a cylindrical member and including an inner peripheral surface that surrounds an outer periphery of the shoulder member to form an accommodation space for the shoulder member, an annular end face that comes into surface contact with the surface of the workpiece to press the surface, and a chamber portion interposed between the inner peripheral surface and the end face, recessed in a diameter-increasing direction with respect to the inner peripheral surface, and opening toward the workpiece, the friction stir spot welding method comprising: a welding step of forming a build-up portion on the workpiece by friction-stirring the workpiece by using the pin member and the shoulder member, while pressing the surface of the workpiece with the end face of the clamp member, and filling a portion of the workpiece being softened in the chamber portion by press-fitting at least one of the pin member and the shoulder member into a plastic flow portion of the workpiece; and a pressing step of pressing a surface of at least one of a region of the build-up portion which is frictionally stirred by the shoulder member and an adjacent region of the build-up portion which is adjacent to the friction-stirred region with the end face of the clamp member with the pin member and the shoulder member retracted into the accommodation space. a welding step of forming a build-up portion on the workpiece by friction-stirring the workpiece by using the pin member and the shoulder member, while pressing the surface of the workpiece with the end face of the clamp member, and filling a portion of the workpiece being softened in the chamber portion by press-fitting at least one of the pin member and the shoulder member into a plastic flow portion of the workpiece; and a pressing step of pressing a surface of at least one of a region of the build-up portion which is frictionally stirred by the shoulder member and an adjacent region of the build-up portion which is adjacent to the friction-stirred region with the end face of the clamp member with the pin member and the shoulder member retracted into the accommodation space.