Abstract: A inorganic-organic composite foam having a structure comprising phosphoric acids and a blowing agent for phosphoric acids, wherein the brittleness is improved by urethane prepolymer having NCO groups. The foam is produced by foaming and curing by mixing each component of phosphoric acids, a blowing agent for phosphoric acids, urethane prepolymer having NCO groups, water, and, if necessary, an inorganic filler. A foam of phosphoric acids is inherently brittle, but the brittleness is improved, and flexibility and rebound resilience can be provided. The foam of the present invention is effectively used for the applications, for example, heat insulating materials of external panel or internal panel which require the fire preventive property of train, automobile, housing, building etc.