To provide a method of producing a fiberboard having superior water resistance and fire resistance, and large physical strength using organic natural fiber.

There is provided the method of producing the fiberboard having improved water resistance, fire resistance and physical strength, in which organic natural fiber 4 such as fiber of wood, cotton, coconut fiber, and bagasse squeezed waste is dipped in a silicate soda solution and then dried, a magnesium chloride solution is sprayed to react in the organic natural fiber 4, and crystal of magnesium silicate 6 which is insoluble in water and incombustible is produced.