We reported a case of aneurysmal bone cyst originating in the left 4th rib. This disease originating in a rib was recorded in only 18 cases in Japan in the literature. The patient was a 16-year-old man and he was first found to have an abnormal shadow in the left upper lung field on the chest roentgenogram. In this case, MRI was very useful to show internal septation of the tumor. We performed a thoracotomy through the left 4th intercostal space with an anteroaxillary skin incision and removed the tumor and the 4th rib completely. Pathological findings showed multiple cysts filled with blood and fibrous trabecullae containing osteoid tissue and multinucleated giant cells, confirming the diagnosis of aneurysmal bone cyst. There has been no evidence of recurrence during the 9 months postoperatively.