PURPOSE: To produce stably titled surface-smooth material within a short time, by impregnating a substrate with a solution of a polyurethane elastomer containing a polyoxyalkylene-modified dimethylpolysiloxane, coagulating the polyurethane elastomer in a coagulation bath and washing and drying the product.

CONSTITUTION: A surface-smooth porous sheet-form material is obtained by a wet membrane-forming process comprising: coating or impregnating a substrate with a solution prepared by mixing a solution of a polyurethane elastomer (preferably, linear polyurethane or polyurea-urethane) with not more than 20wt%, based on solids of the polyurethane elastomer solution, polyoxyalkylene-modified dimethylpolysiloxane of the formula, wherein R is H or a 10C or lower alkyl, R1 and R2 are each a 4C or lower alkylene and (x), (y), (m) and (n) are 0 or positive integers satisfying the relationships: 3≤x+y≤50 and 5≤m+n≤100, and giving an average MW of 600W6,000 to the compound, introducing the substrate into a coagulation bath to coagulate the polyurethane elastomer and water-washing and drying the substrate.