We report an extremely rare case of adenocarcinoma, NOS (cystadenocarcinoma) ; a low-grade malignant salivary gland tumor arising in the mandible. A 64-year-old man visited our hospital because of a painless swelling in the right mandibular gingiva. The lesion appeared radiographically as a well-demarcated multilocular radiolucent area, similar to an odontogenic cystic lesion, in the right side of the mandible. There was no cervical lymphadenopathy. A segmental mandibulectomy was performed under general anesthesia, and the final histopathological diagnosis was adenocarcinoma, NOS (cystadenocarcinoma) . There was no evidence of recurrence or metastasis 51 months after the operation. However, 66 months after surgery, the patient passed away due to double cancer of the lung.