Abstract: In an open joint structure for the prevention of water leakage an isobaric space is formed which has no difference between pressure in the space and ambient pressure. The open joint structure contains at least one groove 11 extending in the vertical direction formed on the exterior side of a vertical edge of a precast concrete curtain wall and which faces an opposing groove 11 of an adjacent precast concrete curtain wall. A heat barrier plate 10 with cutout portions on through holes 13 is mounted between the side walls in the grooves, thereby dividing the isobaric space into a fire-blocking isobaric space (A) between the exterior side of the wall and the heat barrier plate 10 and a radiant heat-blocking isobaric space (B) between the heat barrier plate 10 and a pair of preformed sealants 20, 20. This structure exhibits both a fire-resistant function and a watertight function in an open construction method.