To easily manufacture a product having excellent external appearance with satisfactory mold releasability by casting composition containing SiO2-Al2O3 reactive inorganic powder, alkali metal silicate and water in a mold at a specific contact angle, and thermally curing it.

SiO2-Al2O3 reactive inorganic powder contains 5 to 85 wt.% of SiO2, 90 to 10 wt.% of Al2O3 and alkali metal silicate M2O-nSiO2 (one or more types of metal selected from M=K, Na and Li). In this case, if a value of the n is small, dense foam cannot be obtained, while if it is large, density of water solution is raised so that mixing becomes difficult, and hence the n is 0.05 to 8 or further preferably 0.5 to 2.5. The water may be added independently as alkali metal silicate water solution. If a contact angle of a foam material 1 is large, small foam is generated, while if it is small, its surface is notched so that its external appearance is deteriorated. The angle is preferably 60 to 165 degrees or further preferably 65 to 150. After foam heating, the external appearance of the inorganic foam 2 after cured and mold released does not appear a fault.