Acute mediastinitis is a life-threatening condition, and standard treatment is percutaneous drainage and/or surgical treatment with antibiotics. A 58-year-old male diagnosed with acute mediastinitis originating from pancreatic fistula, detected only by endoscopic-ultrasound. We inserted a transesophageal nasocystic tube under endoscopic ultrasound guidance and successfully controlled the infection. The root of the main pancreatic duct for pancreatic juice drainage was recanalized using a duodenal endoscope. Endoscopic diagnosis and treatment by ultrasound have improved significantly; therefore, clinicians should know the potential of these procedures. The results of the present case may aid in the future treatment of patients with acute mediastinitis.