PURPOSE: To provide a method for producing a lightweight inorganic building material capable of preventing crush of inorganic lightweight aggregate and simultaneously sufficiently suppressing spring back phenomenon due to organic lightweight aggregate and thereby capable of making the weight sufficiently lightweight and of controlling the shape.

CONSTITUTION: This method for producing the lightweight inorganic building material is constituted of steps carried out by blending 100 pts.wt. blend consisting of a cement, a siliceous raw material and a reinforcing fiber with 10-30wt.% inorganic lightweight aggregate and 1-5wt.% crushed body of a thermoplastic synthetic resin crushed to 0.3-1.2 mm particle diameter as an organic lightweight aggregate and adding water thereto, kneading the blend, subjecting the kneaded material to extrusion molding under ≤15kg/cm2 extrusion pressure to provide a molding and curing the molding in an autoclave. The objective lightweight inorganic building material is produced by mutually compensating defects of inorganic lightweight aggregate and organic lightweight aggregate by combined use of both aggregates.