Abstract: A nonflammable building material comprising 3 to 20 parts by weight of a finely divided mineral about 10,000 cm.sup.2 /g in Blaine value and 0.3 to 0.8 in apparent density and 100 parts by weight of a mixture admixed therewith and composed of 20 to 60% by weight of a finely divided aggregate wholly passing through a 4-mesh screen and containing not more than 5% of particles of minus 200 mesh size, 1.0 to 7.0% by weight of a pulp, 0.05 to 2.0% by weight of an organic fiber and the balance cement and siliceous sand in a CaO/SiO.sub.2 mole ratio of 0.5-1.0. The nonflammable building material is produced by a process comprising the steps of preparing a starting mixture of the above composition, spreading the mixture on a belt conveyor, causing water to penetrate into the spread mixture, shaping the wet mixture under pressure and curing the mixture in an autoclave.