Acute mediastinitis is a life threatening disorder that causes severe morbidity in affected patient. We experienced a case of acute mediastinitis in a 56 year-old man as a result of swallowing a fish bone (Order Perciformes, Family Pomacentridae). Conservative treatment, including starvation therapy and the injection of antibiotic drugs, were successful in treating this patient. Although the optimal treatment for acute mediastinitis remains controversial, it is important to choose the appropriate strategies when encountering cases of acute mediastinitis.