Friction stir spot-welding is performed to join together a stack of a plurality of metal foils and a metal sheet by using a double-acting type rotary tool including a hollow member and a pin member which are movable independently of each other in their axial directions, and a clamp member which is fitted on an outer circumferential surface of the hollow member and which is movable in its axial direction. While an outer annular part of a portion to be friction-stirred is pressed with the clamp member, the pin member is inserted into the above-described portion and the hollow member is retracted, whereby a metal mass is accommodated within a space formed between the above-described portion and the hollow member. Then, the hollow member is advanced to press the metal mass into a hole formed by retraction of the pin member, whereby the hole is filled with the metal mass.