Cystadenocarcinoma is an uncommon malignant salivary gland tumor that rarely occurs in the retromolar region. In this paper, we report a case of cystadenocarcinoma that caused considerable resorption of the mandible. The patient was a 79-year-old woman who had a painless swelling, measuring 25 × 25 mm, in the left retromolar region. Panoramic radiography revealed a multiple cystic lesion from the molar to ramus of the left side of the mandible. The tumor was excised by mandibular marginal resection. There was or recurrence or metastasis during 3 years of follow-up.