To solve environmental problems (problems of waste and noxious chemicals, a global warming issue, air and water pollution problems, etc.), and to minimize harmful effects on photographic properties.

A biodegradable flexible sheet 1 is unbleached kraft paper using bast fibers of kenaf as a sort of non-wood natural fiber pulp and having 50 g/m2 basis weight. A hydrophilic colloidal layer 2 is formed on both faces of the sheet 1 by coating with acid treated gelatin containing 5 mass% aluminum hydroxide, 3 mass % synthetic silica as a blocking inhibitor and 1 mass% aldehydes as a hardening agent and having an isoelectric point of 6.0 by a Gate roll coating method in 8 g/m2 dry mass.