To prevent a backfill from flowing out by joint-sealing caissons adja cent to each other without degrading the strength of the caissons in an impervi ous device of a caisson structure to joint-seal the adjacent caissons with a sealant.

Guides 4, 4 are longitudinally provided on side surfaces 2, 2 opposite to each other from an upper end to a lower end of caissons 1, 1 to be provided underwater adjacent to each other in two rows, i.e., forward and rear rows. After these caissons 1, 1 are provided underwater, a tubular bag 7 is provided underwater by the guide of total four guides 4, 4, 4, 4 of the caissons 1, 1 adjacent to each other, a concrete is filled in the bag 7 to swell the bag, and the bag 7 is brought into contact with at least the side surfaces of the caissons 1, 1 adjacent to each other.