An 86-year-old female with a history of right rotator cuff injury was admitted for arthroscopic shoulder surgery under general anesthesia. There were no remarkable immediate postoperative complications. However, while recovering in the general ward, she developed dyspnea with hypoxia. She was immediately treated with oxygen, and antibiotics after pneumomediastinum was confirmed on both chest x-ray and chest computed tomography. Subcutaneous emphysema on either face or neck followed by arthroscopic shoulder surgery was common, but pneumomediastinum with hypoxia is a rare but extremely dangerous complication. Thus we would like to report our case and its pathology, the diagnosis, the treatment and prevention, with literature review.