Galectin 9, which is an immune function-controlling factor belonging to a new class that is different from known cytokines and is expected to be used as a therapeutic drug for various immune-related diseases such as an autoimmune disease, has disadvantages (such as high sensitivity to protease and poor solubility) to be solved in developing therapeutic drugs. Therefore, there is a need to overcome these disadvantages. Chemically modified galectin 9, prepared by conjugating a polymer (for example, poly(ethylene glycol: PEG) to galectin 9, in particular, stabilized galectin 9, is verified to exert higher solubility with improvements in pharmacodynamics and pharmacokinetics (for example, prolongation of action potential duration, such as prolongation of blood half-life and others) and in suppression of hemagglutination and thus has excellent characteristics as a drug.