Colonoscopic polypectomy is a commonly performed procedure in Korea, subsequently prevents colon cancer. The increase of therapeutic colonoscopy, polypectomy, results in inevitably various complications including bowel perforation that is rare and requires emergent decision of treatment modalities and timely treatment. The bowel perforation is usually diagnosed on the basis of pneumoperitoneum on simple X-ray images. Surgery is the treatment of choice for most cases of colonic perforation. A 58-year-old woman was referred to the gastroenterologist for endoscopic mucosal resection (EMR) of laterally spreading flat adenoma in the sigmoid colon. The EMR was performed with flexible and insulated tip knives. The patient was hospitalized for close observation of post-EMR complications. On the day of the polypectomy, simple X-ray images were taken to rule out colonic perforation, which revealed a large amount of intracolonic gas, but no free air in the subphrenic area. The next day of the procedure, subcutaneous emphysema was observed by the clinicians. Then abdomen computed tomography (CT) scan was performed, which showed diffuse bilateral retroperitoneal air extending to the mediastinum without intraperitoneal gas. The patient was discharged from the hospital after medical treatment and supportive care. We herein report a case of colonoscopic perforation in which pneumomediastinum, subcutaneous emphysema and peumoretroperitoneum occurred in the absence of pneumoperitoneum. The perforation was successfully treated with medical treatment and endoscopic clipping. Colonoscopists should not exclude colonic perforation even though free air on simple X-ray images is not visible.