PURPOSE: To prevent the fluctuation in density.

CONSTITUTION: The die consisting of a die 2 having a recessed part 3 on a mold surface, a punch having a projecting part corresponding to this recessed part on the mold surface and a master mold enclosing the punch and the die is used. A molding material A is first charged into this die 1 and is deposited on the mold surface of the die 2. The molding material A is then compressed by the punch to form a preform having a uniform thickness. The charging of the molding material A is executed after a film 7 is flatly held on the mold surface of the die 2 in such a case.