Adenosquamous carcinoma is an uncommon malignant tumor, and histopathologically shows components of adenocarcinoma and squamous cell carcinoma. We report a case of adenosquamous carcinoma in the maxillary sinus associated with an abnormally high level of serum carcinoembryonic antigen (CEA). A 78-yearsold man visited our clinic because of swelling of the left side of the buccal and palatal regions. At presentation, there was a diffuse extraoral swelling extending from the ala of the nose to the infraorbital region, and intraoral swelling and induration in the gingivo-buccal sulcus. A computed tomographic (CT) scan revealed a solid mass in the maxillary sinus, with destruction of the front wall of the maxillary sinus. The serum CEA level was abnormally elevated (50.4ng/ml); however, no distant metastasis was detected. Biopsy was performed under the clinical diagnosis of a malignant tumor of the maxilla. Histopathological examination suggested an adenosquamous carcinoma. Suprahyoid neck dissection (Left-side), ligation of external carotid artery, and subtotal maxillectomy were performed with the patient under general anesthesia. The final histopathological diagnosis was adenosquamous carcinoma. One month after the operation, CEA gradually decreased to 8.9ng/ml. One year after the operation, CEA level was 7.6ng/ml, with no apparent evidence of recurrence.