An inorganic constructional board used for a wall foundation member and the like, which comprises: a middle layer portion (1a) including inorganic foam as its main body and suitable amounts of a fiber material and a bonding agent added to the foam; and outer layer portions (1b) provided on the front and rear surfaces of the middle layer portion (1a), including mineral fibers and an inorganic powder material as its main body and a bonding agent added to them. The specific gravity of the middle layer portion (1a) is made to be 0.4 or therebelow and the specific gravity of the outer layer portion (1b) is made to be 0.7 or thereabove, so that the inorganic constructional board is made lighter than a gypsum board and can provide performance equal to or higher than that of the gypsum board. This inorganic constructional board (1) is manufactured in such a way that a wet inorganic mat as being the under-layer (1b) of the aforesaid construction is made, the middle layer portion (1a) of the aforesaid construction is formed on the under-layer (1b) by a dry method or a wet method, the wet inorganic mat as being the upper layer portion (1b) of the aforesaid construction is superposed on the middle layer portion (1a), and these laminated components are pressed to be integrated, heated and dried.