To realize biodegradable plastic forms for an exterior wall capable of forming an irregular surface on an exterior-wall ground at a low cost.

Several number of sheathing boards manufactured by foam- molding biodegradable plastic so that surfaces on the sides to be finished are formed in the irregular surfaces 5 in which a plurality of fine projections 3 are formed are assembled by using a clamping means, and the biodegradable plastic forms 1 for the exterior wall are expansion-molded in desired shape and size. The biodegradable plastic forms 1 for the exterior wall often used for executing the exterior wall are used as a footing permanent frame when a foundation such as a footing, a footing beam or the like is executed finally.