A method of producing an inorganic substance board, wherein after preparation, such as by use of paper making process, the board is hardened in an autoclave with saturated steam at a high temperature, high pressure and for a suitable length of time, and after hardening, subjecting the board to one or more drying cycles comprising a vacuum treatment at 60 to 300 Torrs, for a suitable length of time to enable the core of the board to reach a suitable temperature, the application of saturated steam at 2 to 12 Kg/cm.sup.2 pressure and 120 to 190.degree. C temperature for a period of time sufficient to allow the core to reach a suitable temperature. The number of cycles is preferably 10 or less. Advantageously, the inventive drying treatment prevents efflorescence and warping of the boards.