Diverticulum of the vermiform appendix is a rare disease and only 66 cases have been reported in Japan. In this paper, a case of the disease is described. A 23-year-old woman was admitted to the hospital because of right lower quadrant pain. Acute appendicitis was suspected and operation was performed. A tumor with a size of the thumb was palpated on the mesentery of the vermiform appendix. Appendectomy was carried out. Gross examination of the appendix revealed an absence of inflammation but a tumor formation with partial abscess in the appendiceal submucosa, and some structure seeming diverticulum. Histopathologically 4 diverticulum ducts and abscess due to diverticulitis were observed. These diverticula were diagnosed as pseudodiverticula. In the treatment of appendiceal diverticulum, operation is, of course, indicated, if diverticulitis developed. If an asymptomatic diverticulum is accidentally found by barium enema or in the operative course for other reason, appendectomy is recommended, because it is less invasive surgical procedure and appendiceal diverticulum has a high risk of perforation compared to colic diverticulum.