To solve the problems resulting from the marks of the air holes for vacuum suction.

In conducting a press by pressing the molding die 3 against the surface of the inorganic plate 1, the surface of the inorganic plate is sucked by vacuum to the molding die side through the air holes 4 prepared in the molding die, then pressurized air with a predetermined pressure is supplied to the air holes with the mold clamping kept, and thereafter the mold opening is conducted to release the inorganic plate from the molding die. The marks of the deaeration caused in the inorganic plate upon the press molding by the air holes for deaeration are eliminated by pushing them back by supplying the pressurized air to the air holes while the mold clamping is maintained.