To prepare a thick inorg. sheet material by a method wherein a mixture slurry of a hydraulic inorg. material, water, aggregate, etc., is subjected to a paper machine process two times to make two mats and the mixture slurry of a hydraulic inorg. materials, water and aggregate is applied thinly on one of the mats and another mat is laminated thereon to be subjected to pressing molding.

An endless filter cloth 23 is moved in the arrow mark diffraction by receiving a rotational powder from a guide roller 22 and a slurry 29 contg. hydraulic inorg. Material in raw material containers 26-28 adheres gradually thereto to form a mat 30 with a definite thickness, which is transferred to a making roll 24. The transferred may 30 is released from the making roll 24 by a cutting-out device 25 and is cut into a definite length. Thereafter, the surface of the may 30 is coated thinly with the slurry 29 and them, a mat 32 made into a sheet is placed from the top to make a thick laminated may, which is them pressing-molded to prepare a sheet material.