A 70-year-old woman underwent conservative treatment for abscess-forming appendicitis. A contrast-enhanced abdominal computed tomography(CT)revealed a cystic lesion at the appendiceal base while the abscess had disappeared posttreatment. With the diagnosis of a low-grade appendiceal mucinous neoplasm(LAMN), a laparoscopic-assisted ileocolic resection was performed. The appendix was distended with mucus in the lumen. Histopathological examination showed that the tumor cells were more atypical than that of low-grade appendiceal mucinous neoplasm. There were no findings of adenocarcinoma, such as invasive growth. Therefore, high-grade appendiceal mucinous neoplasm(HAMN)was diagnosed. HAMN is a term advocated by Carr et al in 2016 to classify appendiceal mucinous neoplasm and was noted as a new category of mucinous tumors among appendiceal epithelial tumors in the fifth edition of the 2019 revised WHO classification. HAMN is a rare disease and only one case has been reported in Japan. It has not yet been noted in the Japanese Colorectal Cancer Treatment Manual. Here, we report this rare case with a review of the study.