We report a case of 59-year-old man of descending necrotizing mediastinitis (DNM) secondary to peritonsillar abscess. A 59-year-old man with diabetes mellitus was admitted to a local hospital because of cervical swelling related to a peritonsillar abscess. Despite administration of antibiotics, swelling of the neck, dysphagia and dyspnea deteriorated. Therefore he was urgently undergone a tracheotomy and transferred to our hospital by an ambulance. The surgery consisted with neck and anterior mediastinal drainage through neck and cervical collar incision. Culture of drainage fluid showed clostridium difficile. On postoperative day 5, we started hyperbaric oxygen therapy (HBOT). After lavage and HBOT, the patient improved by degrees, and discharged on postoperative day 82. DNM is a rare but serious complication of otopharyngeal and deep neck infection that spreads down to the mediastinum through the cervical-facial planes. Its mortality rate remains high even with aggressive surgical drainage and appropriate antibiotics. Our patient was successfully treated with urgent surgical drainage, antibiotics and HBOT. HBOT might be of great value as an adjunctive management to control this fatal infection.