PURPOSE: To provide a sufficiently lightweight building material holding homogenous strength by preventing the segregation of lightweight aggregate by supplying a predetermined low con. slurry of a raw material for a lightweight inorg. building material containing foamed synthetic resin particles to an absorbing dehydration molding belt from a supply device moved in the direction reverse to the moving direction of the molding belt to immediately dehydrate the same.

CONSTITUTION: A slurry 4 to be used is prepared by adding water to a compounded material of an inorg. powder material, lightweight aggregate such as foamed synthetic resin particles and a lightweight inorg. building material composed of reinforcing fibers to adjust the concn. of the compounded material to 30-50%. When this slurry 4 is supplied to a dehydration molding belt 1, a supply device 3 moving in the direction opposite to the moving direction of the molding belt 1 is used. Due to this constitution, the relative supply speed of the slurry to the molding belt 1 is increased and a layer is formed while the lightweight aggregate is fixed at an early stage by an absorbing dehydrator 2. Therefore, the float segregation of the lightweight aggregate to the surface of the layer is prevented and the dispersibility thereof is drastically improved.