A substrate for flexible device. The substrate has a nickel-plated metal sheet having a nickel-plating layer formed on at least one surface of a metal sheet or a nickel-based sheet, and a glass layer of an electrically-insulating layered bismuth-based glass on a surface of the nickel-plating layer or the nickel-based sheet. An oxide layer having a roughened surface is formed on the surface of the nickel-plating layer or the surface of the nickel-based sheet, and the bismuth-based glass contains 70 to 84% by weight of Bi2O3, 10 to 12% by weight of ZnO, and 6 to 12% by weight of B2O3. Also disclosed is a method for producing the substrate for flexible device, a substrate for an organic EL device, a sheet used as a substrate for flexible device, a method for producing the sheet and a bismuth-based lead-free glass composition.