A 73-year-old woman who drank 1 L of a liquid cleansing agent for colonoscopy complained of severe abdominal pain and distention. An emergency CT study and colonoscopy revealed a giant impacted fecalith in the sigmoid colon which caused colonic ileus. The fecalith was removed colonoscopically. Barium enema study showed a giant sigmoid diverticulum, 3 cm in diameter, so a laparoscopy-assisted sigmoidectomy was performed. At operation, the giant sigmoid diverticulum was located on the antimesenteric border. Histopathologic study revealed a lack of proper muscle layer in the diverticulum. There have been only 28 reported cases of giant colonic diverticulum in Japan, which are reviewed here. The appearance of a balloon in CT study is an important sign in the diagnosis of giant colonic diverticulum.