It is an object of the present invention to a method whereby a humoral immune response is induced more efficiently in producing an antibody against an antigen protein by gene immunization. A fusion gene composed of a gene encoding the full-length of a part of the antigen protein or a gene encoding a chaperonin subunit or a chaperonin subunit linkage linked thereto is administered to express the fusion gene in the animal, thereby inducing a humoral immune response to an antigen protein by administering. An example of the chaperonin includes Escherichia coli GroEL. There is also provided with a composition for immunization, a method for producing an antibody, a method for producing a hybridoma, and a method for producing a monoclonal antibody.