To improve releasability while preventing joint bend or the like from being generated by uniformly and smoothly releasing a green board molded product having an irregular pattern formed on a surface from a mold in a press molding process for manufacture of an inorganic board.

In a press molding process of forming a required irregular pattern by pressing a mold 4 onto a surface of an inorganic green board in a pressing machine 1, the surface of the inorganic green board is vacuum sucked to the mold side. Thereafter, when a green board molded product 3a on which the irregular pattern is formed is released fro the mold by blowing air onto the surface, vibration is applied to the mold.