Abstract: An inorganic/organic composite foam that has a foam structure obtained from a combination of a phosphoric acid compound and/or a sulfuric acid compound with a blowing agent therefor, has reduced brittleness due to a cured material of an urethane prepolymer having NCO groups, and contains a powdery boric acid compound, and a process for producing the same. The foam is not only reduced in brittleness but also improved in foam strength after combustion due to the incorporation of the powdery boric acid compound while retaining inherent foam properties and inherent low quantities of heat of combustion and smoking in combustion. It is used in exterior wall panels, heat insulating materials, sound insulating materials, fireproof covering materials, lightweight aggregates, filling materials for cavities, and the like, which are required to have fire-proofing performance.