A protein is efficiently nitrosylated with nitrogen monoxide by merely mixing S-nitrosoglutathione as a nitrogen monoxide donor with a protein solution containing a stabilizing agent comprising at least one compound or a combination of plural compounds selected from an N-acetylamino acid, a fatty acid, and a fatty acid salt. The method that enables efficient NO addition to a cysteine residue in un-nitrosylated protein without changing the structure of protein and hence provides NO to a living organism.