|| A 64-year-old woman was admitted to our hospital with lower abdominal pain. Routine laboratory values were unremarkable except for the white blood cell count (15,000/micro litter) and the C-reactive protein (CRP) value (22.5 mg/dl). A Computed tomography (CT) scan revealed air collection in the middle of the anterior pararenal space. One day later, CT revealed air collection in the anterior pararenal space spread to the right side and abscess in the sigmoid mesentery. Because an intramesocolic perforation of the sigmoid colon was suspected, an emergency operation was performed. Abscess formation was recognized in the sigmoid mesentery, and sigmoidectomy including the contaminated mesentery and Hartmann.s procedure were performed. The perforation was 3 cm in diameter, and some diverticula were present in the vicinity of the perforated site. The specimen microscopically revealed perforation at the edge of the diverticulum in association with sudden disruption of the proper muscle layer. Based on pathological findings, intramesocolic diverticular perforation of the sigmoid colon was diagnosed. The present case is a very rare condition. However, it was possible to make a diagnosis preoperatively by detecting air collection in the anterior pararenal space on CT scan. If a sigmoid perforation occurs between the leaves of the mesocolon, air extends into the root of the sigmoid mesocolon and within the anterior pararenal space.