PURPOSE: To raise the sealability against inner pressure of a tubeless double caisson type water-sealing rubber by constituting bonding faces at both sides of the back, to which upper and lower water sealing rubber members having convexed sectioned shape are to be vulcanized and bonded, to mesh with each other.

CONSTITUTION: A tubeless double caisson type water-sealing rubber 1 into which high-pressure water is to be introduced from a tube 7 when the gate is interrupted from water and which is to be provided at the peripheral edge of the diversion channel of a dam is formed by vulcanizing and bonding upper and lower water-sealing rubber parts 2 and 3 having a convexed cross section form. In this case, a mutually meshing bonding face of a serrated or crank shape is provided for the bonding faces 4 and 5 of both sides of the backs of the rubber parts 2 and 3. Accordingly, the vulcanized bonding area is increased and its sealability in said part is thus raised.