A 79-year-old man presented with chest pain, and urinary antigen tests for Legionella pneumophila (ImmunoCatch^® Legionella and Ribotest^® Legionella) were negative on admission. The next day, rapid respiratory failure suggested Legionella pneumonia, and levofloxacin was added. Since a lung infiltration shadow appeared on the other side as well on day 4, non-infectious diseases were considered, and steroid therapy was started. Urinary antigen tests for Legionella pneumophila became positive on day 5. In the present case, retesting with Ribotest^® Legionella, which could be negative early after the disease onset, was useful for diagnosing Legionella pneumonia, which led to the discontinuation of unnecessary steroid treatment.