A method includes a forming step of forming a first metal separator and a second metal separator each including a protruding sealing bead portion and a protruding stopper bead portion; a joining step of joining surfaces of the first metal separator and the second metal separator on sides opposite to sides on which their respective sealing bead portions protrude, and attaching seal members in an extension direction or on distal ends of the sealing bead portion; and a preload applying step of applying preload to a bead seal section formed of one pair of the sealing bead portions and the seal members and a stopper section formed of one pair of the stopper bead portions in a height direction thereof, thereby plastically deforming the bead seal section and the stopper section simultaneously.