An 83-year-old woman on hemodialysis was hospitalized due to high fever and vomiting. On admission, she was diagnosed as having Influenza A virus infection based on examination by influenza virus antigen kit. Despite treatment with amantadine and β-lactum antibiotics, pulmonary infiltration and hypoxemia developed. Therefore, we carefully examined her serum and she was subsequently as having diagnosed Legionella infection based on positive findings using a serum Legionella antigen kit. She rapidly recovered from Legionella pneumonia after administration of erythromycin, rifampicin and levofloxacin. Serum Legionella antigen is of enourmous value in identifying Legionella infection in a hemodialysis patient.