A 76-year-old woman with right mandibular gingival cancer was scheduled for surgery. A percutaneous tracheostomy kit was used for tracheostomy under intravenous sedation. After puncturing the cricothyroid membrane, a dilator was inserted along a guidewire. Bucking was observed at the time of insertion of the dilator, despite intratracheal lidocaine spray applied before insertion. After that, the tracheostomy tube was inserted, but no capnographic waveforms appeared when the tube was connected to the anesthesia circuit. Direct macroscopic observation revealed a perforation extending from the posterior wall of the trachea to the anterior wall of the esophagus, which prompted us to request assistance from a thoracic surgeon for treatment before reinserting the tracheostomy tube. After verifying capnographic waveforms on the monitor, anesthesia was induced. Because arterial oxygen saturation was 96% under the administration of pure oxygen, chest radiography was conducted revealing a right pneumothorax. A chest tube was inserted and the patient transported to a nearby general hospital. We suspect that pneumothorax was induced when the integrity of the mediastinal pleura was compromised by mediastinal emphysema because of the injury to the posterior tracheal wall complicated by bucking at the time of insertion of the dilator.