A flexible membrane dam is provided which can be installed easily and in which a concentration of stress at a time the dam is made to stand is less than in conventional structures. Mounting hardware for the dam is also provided. When a fluid such as air is supplied to an interior of a flexible membrane, the flexible membrane inflates and stands, and tension acts on the flexible membrane . At inclined surface portions, a vicinity of an outer peripheral edge of the flexible membrane is, in a convex and concave shape, nipped and fixed between an upper pressing hardware and a lower pressing hardware . At the upper pressing hardware, a ratio (AE1/AE1').times.100 of a length AE1 from a point E1 of a concave portion to a point A, to a length AE1' from a point E1' of a convex portion to the point A, is 85 or more. Namely, the length AE1' is close to the length AE1. Thus, at an inflated main body side of the flexible membrane, local drawing up of the flexible membrane by the upper pressing hardware is suppressed, i.e., a concentration of stress at the flexible membrane when inflated is suppressed. The flexible membrane can be stand up in a planar shape from end portions of the upper pressing hardware.