Abstract: It is intended to provide an immunogen and a composition for immunological use by which a sufficient immune response is induced even by an antigen protein which has been considered unavailable to induce any effective immune responses for various reasons, and a method of producing an antibody using the same. The immunogen containing a fusion protein including the full-length or a part of a desired antigen protein and a folding factor or its subunit linked thereto via at least one peptide bond. Owing to the function of the folding factor, the antigen protein contained in the fusion protein is obtained as a soluble protein having a correct stereostructure. It is also possible to protect the antigen protein and from rapid decomposition in the blood of an animal having been inoculated therewith. The immunogen induces a higher immune response for an animal than an antigen protein alone. Examples of the folding factor include chaperonins and PPIases.