A case of descending necrotizing mediastinitis that was treated by mediastinoscopic drainage is reported. The patient was a 56-year-old diabetic woman. A hypopharyngeal abscess extended to the mediastinum through the neck. No septic condition was noted. Chest CT showed that the abscess reached 4 cm below the tracheal bifurcation. Pus was drained under direct observation by mediastinoscopy, and a drain was placed in an appropriate position. After operation, lavage was performed through the drain, and cure was achieved on the 42nd postoperative day. This technique should be considered as surgical treatment for descending necrotizing mediastinitis in the absence of serious complication such as sepsis, because it has a more reliable drainage effect than the conventional transcervical method, and because it is less invasive than thoracotomy.