Although acute appendicitis is one of the most common surgical diseases, very rare but serious complications can occur including retroperitoneal abscess complicated by acute appendicitis. The early diagnosis of retroperitoneal abscess is often difficult and delayed because its clinical course is often insidious and does not present with typical symptoms of acute appendicitis. We experienced a 65-year-old male patient who was diagnosed with acute appendicitis with retroperitoneal abscess. As the retroperitoneal abscess was not fully removed via emergency operation, postoperative percutaneous drainage was performed but not effective. Retroperitoneal abscess can be cured by 2nd operation with retroperitoneal approach. We report here an unusual case of retroperitoneal abscess complicated by acute appendicitis.