To realize the oozing of excess water content in a short period of time and high mechanical strength and beautiful surface quality for an inorganic hardening composition by a method wherein a hard-flowing material is spreadingly supplied on the forming surface of each retainer plate and, at its compressing with a pressing machine, pressing pressure is applied and released a plurality of times.

Each retainer plate 3 makes a pair with a hot plate 6 having a heating device. In a cavity made between the retainer plate and the hot plate 6, a hard-flowing material 2 spreadingly supplied on the forming surface of the retainer plate 3 is formed. In this case, the shape of the forming surface of the retainer plate 3 is a flat plate type, a wavy type or the like. A pressing machine 7 transmits increasing and decreasing pressures to the retainer plate 3 and the hot plate 6 in each stage. The increasing and decreasing pressures applied to the hard-flowing material 2 pinched between the retainer plate 3 and the hot plate 6 in each stage has a pattern to be pressurized first of all by a large amplitude, then by gradually decreasing amplitudes and finally by a certain fixed pressure or the like. The magnitudes and the number of times of the application of the increasing and the decreasing pressures depend upon the properties, size and the like of the hard-flowing material 2 to be used.