The patient was a 67-year-old man with diabetes mellitus who had been to a hot spring spa a few days before his admission. The diagnosis of Legionella pneumonia was made using a urinary antigen assay. Intravenous pazufloxacin and oral clarithromycin were started. However, despite these treatments, he developed acute respiratory distress syndrome (ARDS). He was administered the combination of intravenous pazufloxacin and erythromycin, corticosteroid, and sivelestat for two weeks. Then he was successfully recovered. The outcome suggests that treatment with corticosteroid and sivelestat, in addition to a combination of appropriate anti-Legionella antibiotics, should be considered for patients with severe Legionella pneumonia with ARDS.