A manufacturing method of a laminated power storage device disclosed herein includes a step A of preparing an electrode body to which an electrode terminal is connected, a step B of preparing a laminated film, a step C of building a power storage device assembly in which the electrode body is covered with the laminated film and is sealed, and a step D of placing the power storage device assembly in a predetermined reduced-pressure atmosphere. The power storage device assembly has a welding portion in which the laminated films are stacked and welded together in a state in which part of the electrode terminal is protruded to the outside from between the laminated films around the electrode body. A pressure of the reduced-pressure atmosphere in the step D is set so as to guarantee that a weld strength of the welding portion is not less than a predetermined strength.