A 21-year-old female was admitted to our hospital because of high fever, neck swelling, and dyspnea. She was diagnosed as descending necrotizing mediastinitis (DNM) extended from odontogenic infection. On the day of admission, she underwent cervical drainage. Next day, the CT scan showed an abscess below the tracheal bifurcation and bilateral pleural effusion. Mediastinal drainage was performed through a right thoracotomy, and a left thoracic tube was inserted. Anaerobic Peptostreptococcus was found with bacteriological culture. After the mediastinal drainage, bilateral thoracic irrigation was performed through the thoracic tubes. Left thoracic tube was removed on the 8th day and right one was removed on the 20th day after the thoracotomy. She was discharged on the 42nd day. DNM is relatively rare, but it is lethal disease with high mortality. Immediate and sufficient mediastinal drainage is indispensable for the disease.