Inorganic board manufacturing method and inorganic board

In order to provide an inorganic board suitable for achieving high water resistance and a method for manufacturing the inorganic board, the manufacturing method of the present invention includes first to sixth steps. In the first step, a first layer (L1) is formed by stacking a raw material on a receiving plate (B1). In the second step, one end portion (first portion (Ma)) and the other end portion (second portion (Mb)) in the first direction (D1) of the raw material slab (M) including the first layer (L1) are pressed and compressed toward the receiving plate (B1). In the third step, a second layer (L2) is formed on the first layer (L1) by deposition of the raw material. In the fourth step, the exposed surface of the second layer (L2) is planarized. In a fifth step, the raw material slab (M) is cured while being pressed between the receiving plate (B1) and the pressing plate (B2), and a hardened plate (M ') is formed from the raw material slab (M). In a sixth step, the first portion (Ma) is processed to form