To obtain a composition capable of forming a building exterior body low in specific gravity and water absorption, excellent in strength and reduced in a production cost by using a cement, in which a fibrous material is incorporated as an essential raw material, and incorporating a specific lightweight aggregate.

The cement, in which the fibrous material (e.g. pulp fiber) is incorporated, is used as the essential raw material and 10-20wt.% fly as balloon is preferably incorporated as the lightweight aggregate. Since the fly ash balloon has high compression strength of ≥10MPa compared to the conventional aggregate (e.g. perlite) and the 10MPa pressurization is applied to the molding of the building exterior body, the exterior body provided with a sufficient mechanical strength is obtained. Since the fly ash balloon contains 60% SiO2, it causes the Pozzolan reaction with Ca(OH)2 liberated from C3S (basic composition of A-lite) or C2S (basic composition of B-lite) or C3S2H3 (afwilite) as the reaction product, thus the structure of the exterior body is densified.