To provide a composition for prevention, amelioration, or treatment of diabetes mellitus and/or diabetic nephropathy, which is less likely to cause any disadvantages such as side effects, and highly safe. [1] A composition for prevention, amelioration, or treatment of diabetes mellitus and/or diabetic nephropathy, containing one or more dietary fiber selected from the group consisting of a degraded galactomannan, an indigestible dextrin, a polydextrose, insulin, arabinogalactan, dietary fiber derived from corn, a water-soluble soybean polysaccharide, psyllium, and a low-molecular weight sodium alginate, and/or a polyphenol compound.