Legionella longbeachae is a Legionella bacteria often detected in soil, and is known as a rare cause of Legionella infections in Japan. In addition, detection of this Legionella species is often overlooked due to negative results from Legionella urinary antigen tests, which could lead to errors in the therapeutic approach. An 80-year-old woman was admitted to our hospital because of fever and dyspnea. Her blood tests showed elevated white blood cells, increased C-reactive protein and transaminases, and hyponatremia. Chest computed tomography showed dense consolidation in the right lung. We diagnosed Legionella pneumonia because the Legionella urinary antigen test was positive on the day after her admission. The patient was intubated and mechanically ventilated on the third day of hospitalization, because of respiratory failure. However, her condition did not improve and she died on the 10th day after admission. After her death, L. longbeachae was detected from sputum culture from her tracheal tube, and was diagnosed as the causative organism of her pneumonia. L. longbeachae infection reportedly rarely produces positive urinary antigen test results. Our experience suggests that the urinary antigen test using Ribotest Legionella might be able to detect Legionella spp. other than L. pneumophila.