To efficiently receive spary by providing a delta-shaped sponson capable of receiving the spray jumping up by entering all the way from a ship's bottom surface, being adjusted in the outflow direction of the spray and having a width expanded in the stern direction on both-board side outside plates of a stern part.

A sponson 11 is formed in the shape of being adjusted in the outflow direction of spray S jumping up by entering all the way from a ship's bottom surface, and is actually formed in a delta shape capable of receiving the spray S jumping up by entering all the way from the ship's bottom surface and having a width expanded backward. This is because an inflow angle of the spray S is not parallel to ship side outside plates 7 and falls within a specific angle. Thus, the sponson 11 expanded backward as a widening width angle θ of 10° to 15° from the front end is desirable. Particularly, when the widening with angle θ of the sponson 11 is set to 15°, almost all spray S can be received.