The present invention relates to a method of improving storage stability of reduced form of nicotinamide adenine dinucleotide, reduced form of nicotinamide adenine dinucleotide phosphate, or a salt thereof, or ascorbic acid, an ascorbic acid derivative, or a salt thereof, which comprises allowing the substance to coexist with an L-arginine acidic amino acid salt, a composition containing the substance and an L-arginine acidic amino acid salt, a process for producing the composition, and a method of storing the substance in the presence of an L-arginine acidic amino acid salt.
