To provide a building substrate which can be laid even by a layman without requiring the technique of a plasterer, and to construct an exterior wall which has plasticity deformable to a curved surface and is best fit for a roof or the like on a window section or an entrance section.

In production of the building substrate such as the external heat insulating wall material or the floor material having a mesh arranged on an upper surface of a thin mortar layer, base mortar is applied to a heat insulating board, and then the mesh is laid on the mortar, followed by applying surface mortar. Then the building substrate of a four-layered structure consisting of the heat insulating board, the substrate mortar, the mesh, and the surface mortar, is compressed via a reduction roll, and the substrate mortar or the surface mortar is compressively formed into the thin mortar layer having a predetermined thickness.