To prevent deformation of a flexible membrane-made expansive dam by directly or indirectly arranging plural steel plates on the upper surface from the upstream surface in a part or the whole of the lengthwise directional central part of the flexible membrane-made expansive dam.

Steel plates 2 are arranged in a part or the whole at least in the lengthwise directional central part of a flexible membrane-made expansive dam 1. These steel plates 2 are arranged in a plurality in the lengthwise direction of the flexible membrane-made expansive dam 1 so as to reach the upper surface from the upstream surface. The steel plates 2 are directly or indirectly arranged in the flexible membrane-made expansive dam 1. That is, the steel plates 2 are installed in the flexible membrane-made expansive dam 1 by bolts when the steel plates 2 are directly arranged in the flexible membrane-made expansive dam 1. A sheet-like member is fixed to the flexible membrane-made expansive dam 1 after installing the steel plates 2, for example, on the sheet-like member by bolts when the stele plates 2 are indirectly arranged in the flexible membrane-made expansive dam 1. Therefore, the steel plates 2 exhibit a rib-like function to prevent the occurrence of V-shaped deformation.