A 59-year-old man presented with abdominal and left flank pain. The symptom had started 30 days before as an acute nephrolithiasis, which had worsened despite conservative management. The abdomen was slightly distended and tender over the lower abdomen, without signs of generalized peritoneal irritation. A computed tomography (CT) scan showed an abscess in left para-renal space up to the subphrenic space and an unexpected pneumomediastinum. An emergency operation was performed, which showed retroperitoneal diverticulitis perforation of the sigmoid descending junction with abscess formation. A segmental resection of the diseased colon and end-colostomy was performed (Hartmann's procedure). However, the patient's condition progressively deteriorated, and he died of sepsis and multi-organ failure on the 5th postoperative day. Although pneumomediastinum caused by colonic diverticulitis perforation is extremely rare, it could be a life-threatening condition in patients without signs of peritonitis because of delayed diagnosis.