A method of producing a plate stack preform, including preparing a reinforcement stack having first and second reinforcing plates, overlapping first and second plates respectively larger in size than the first and second reinforcing plates on the first and second reinforcing plates, respectively, inserting a weld-preventing conductive plate between side peripheries of the overlapped first and second reinforcing plates, after the inserting operation, simultaneously conducting joining the first plate with the side periphery of the first reinforcing plate and joining the second plate with the side periphery of the second reinforcing plate by lap resistance welding, and then joining side peripheries of the first and second plates together by continuous welding.