A human monoclonal antibody (IgG) against Pseudomonas aeruginosa was established within 3 weeks after fusion of patient lymphocytes and human myeloma WIL2-86 cells. This method is useful in establishing human monoclonal antibodies for infectious bacteria of a variety of antigenicities and allows for use of such antibodies as immunotherapeutic drugs in chronically infected patients. The opsonic effect of the monoclonal antibody we established was 1.8-fold higher than that of commercially available immunoglobulin drugs. Since the established human monoclonal antibody recognizes serotype M, which has not yet been produced, the antibody may be used as a part of immunotherapeutic drug “cocktail”.