Abstract: A method is provided for making a fiber-reinforced slag gypsum cement lightweight hardened product. The method consists essentially of mixing (1) from 5 to 140 parts by weight of aggregates having a maximum size of not larger than 2000 .mu.m to (2) 100 parts by weight of a mixture which comprises 100 parts by weight of Portland cement, (3) from 20 to 350 parts by weight of slag fine powder which has a fineness ranging from 6,000 to 12,000 cm.sup.2 /g as Blaine's specific surface area, (4) from 0 to 20 parts by weight of lime, (5) from 20 to 100 parts by weight of calcium aluminate and gypsum fine powder, (6) 0.01 to 1.5 parts of a setting retardant; mixing water, prefoamed foams and reinforcing fibers with the mixture to obtain a slurry; subjecting the slurry to molding to obtain a molded product; removing the molded product to obtain a hardened product; and curing the hardened product with steam.