A separator welding device which welds a pair of separators, comprising: a conveyance unit that conveys the pair of separators, wherein the conveyance unit includes a pair of stacking drums (210, 220) that superimpose the pair of separators while conveying the pair of separators in a state where the separators protrude beyond the peripheral surfaces of the stacking drums (210, 220); joining heads that are able to move closer to each other and be separated from each other with respect to the pair of separators; joining tips (302, 303) that are attached to the joining heads and join the separators to each other; holding units that fasten the separators, the holding units being able to move back and forth between an advanced position beyond distal ends of the joining tips, and a retracted position where the holding units are retracted from the advanced position, wherein the joining tips (302, 203) and the holding units are arranged on the side of the stacking drums (210, 220); and a joining unit including a biasing member that is provided between the joining head and the holding unit, and biases the holding unit with a biasing force that moves the holding unit towards the advanced position, wherein, as the joining heads move closer to each other with respect to the separators, the holding units come into contact with the separators and are retracted to the retracted position from the advanced position, and the distal ends of the joining tips come into contact with the separators and join the separators to each other, as the joining heads are separated from each other with respect to the separators from a state where the distal ends of the joining tips are in contact with the separators, the distal ends of the joining tips are separated from the separators in a state where the separators remain fastened by the holding units, and, the holding units move forward to the advanced position from the retracted position due to the biasing force of the biasing member, and are separated from the separators, and the lateral edges of the pair of the separators are joined by the joining tips (302, 303) immediately after the pair of separators are separated from the stacking drums (210, 220) in a state where the separators remain fastened by the holding units. a conveyance unit that conveys the pair of separators, wherein the conveyance unit includes a pair of stacking drums (210, 220) that superimpose the pair of separators while conveying the pair of separators in a state where the separators protrude beyond the peripheral surfaces of the stacking drums (210, 220); joining heads that are able to move closer to each other and be separated from each other with respect to the pair of separators; joining tips (302, 303) that are attached to the joining heads and join the separators to each other; holding units that fasten the separators, the holding units being able to move back and forth between an advanced position beyond distal ends of the joining tips, and a retracted position where the holding units are retracted from the advanced position, wherein the joining tips (302, 203) and the holding units are arranged on the side of the stacking drums (210, 220); and a joining unit including a biasing member that is provided between the joining head and the holding unit, and biases the holding unit with a biasing force that moves the holding unit towards the advanced position, wherein, as the joining heads move closer to each other with respect to the separators, the holding units come into contact with the separators and are retracted to the retracted position from the advanced position, and the distal ends of the joining tips come into contact with the separators and join the separators to each other, as the joining heads are separated from each other with respect to the separators from a state where the distal ends of the joining tips are in contact with the separators, the distal ends of the joining tips are separated from the separators in a state where the separators remain fastened by the holding units, and, the holding units move forward to the advanced position from the retracted position due to the biasing force of the biasing member, and are separated from the separators, and the lateral edges of the pair of the separators are joined by the joining tips (302, 303) immediately after the pair of separators are separated from the stacking drums (210, 220) in a state where the separators remain fastened by the holding units.