A fireproof flat glass which is formed from a glass composition classified as soda-lime glass. There has been no soda-lime flat glass having high durability and less susceptibility to softening, although a fireproof flat glass made of soda-lime glass has been desired which can be produce efficiently and has no fear of shortening the life of a glass-smelting furnace. Regulating the soda-lime glass so as to have a specific composition with a softening point of 780 DEG C or higher has made it possible to provide a fireproof flat glass having high durability and less susceptibility to softening.
