The invention provides an ultrathin high-strength flexible sheet that has exceptional appearance, surface smoothness, and feel; and that presents no risk of rubber residues forming even in the case of degradation. An ultrathin flexible sheet is formed from an elastomer composite in which the pores of a porous film (e.g., a film made from stretched porous polytetrafluoroethylene) are filled with an elastomer, and the elastomer layer that covers the surface of the porous film is one micron or less in thickness. Examples of elastomers that may be employed include silicone-based elastomers (e.g., a methyl silicone-based elastomer and a phenyl silicone-based elastomer) and fluorine-based elastomers (e.g., a fluorosilicone elastomer, a fluoroelastomer, and a perfluoroelastomer). It is recommended that part of the surface of the porous film be left exposed. The ultrathin flexible sheet can be made 50 .mu.m or less in thickness.