A door opening/closing device for a double folding door, comprising: a support shaft that foldably connects a first door panel and a second door panel; and a torsion spring provided between the first door panel and the second door panel coaxially with an axial center of the support shaft, an opening being closed when the first door panel and the second door panel are in the unfolded state, and the opening being open when the door panels (16, 18) are in a folded state, and the torsion spring impelling the first door panel and the second door panel in a direction to close the opening, the torsion spring being made from a single wire member and including a first coil portion and a second coil portion in which the wire member is wound in opposite directions to each other coaxially with a spacing therebetween, and a latched member that connects the first coil portion and the second coil portion, the latched member being latched to a first door panel side, an end portion of the first coil portion on an opposite side to the latched member and an end portion of the second coil portion on the opposite side to the latched member being latched to a second door panel side, and the first door panel and the second door panel being impelled in the direction to close the opening by elastic force that tends to restore the first coil portion and the second coil portion to a wind open direction, characterized in that a shaft member is provided fitted at a first end in a longitudinal direction to the first door panel so that it cannot rotate and is rotatably supported at a second end in a longitudinal direction by the second door panel, the shaft member includes a first shaft around which the first coil portion is wound, a second shaft around which the second coil portion is wound, and an attaching member that removably connects the first shaft and the second shaft coaxially so that they can rotate integrally, and the attaching member having a latching portion that latches the latched member to the attaching member; wherein the support shaft is configured so that an outside cylindrical member is provided that covers the portion of the shaft member around which the torsion spring is wound, and the outside cylindrical member is configured so that a portion near a first end in the longitudinal direction is rotatably supported by the first door panel, and a portion near a second end in the longitudinal direction is fitted to the second door panel so that it cannot rotate. a support shaft that foldably connects a first door panel and a second door panel; and a torsion spring provided between the first door panel and the second door panel coaxially with an axial center of the support shaft, an opening being closed when the first door panel and the second door panel are in the unfolded state, and the opening being open when the door panels (16, 18) are in a folded state, and the torsion spring impelling the first door panel and the second door panel in a direction to close the opening, the torsion spring being made from a single wire member and including a first coil portion and a second coil portion in which the wire member is wound in opposite directions to each other coaxially with a spacing therebetween, and a latched member that connects the first coil portion and the second coil portion, the latched member being latched to a first door panel side, an end portion of the first coil portion on an opposite side to the latched member and an end portion of the second coil portion on the opposite side to the latched member being latched to a second door panel side, and the first door panel and the second door panel being impelled in the direction to close the opening by elastic force that tends to restore the first coil portion and the second coil portion to a wind open direction, characterized in that a shaft member is provided fitted at a first end in a longitudinal direction to the first door panel so that it cannot rotate and is rotatably supported at a second end in a longitudinal direction by the second door panel, the shaft member includes a first shaft around which the first coil portion is wound, a second shaft around which the second coil portion is wound, and an attaching member that removably connects the first shaft and the second shaft coaxially so that they can rotate integrally, and the attaching member having a latching portion that latches the latched member to the attaching member; wherein the support shaft is configured so that an outside cylindrical member is provided that covers the portion of the shaft member around which the torsion spring is wound, and the outside cylindrical member is configured so that a portion near a first end in the longitudinal direction is rotatably supported by the first door panel, and a portion near a second end in the longitudinal direction is fitted to the second door panel so that it cannot rotate.