A case of simultaneous malignant melanoma and squamous cell carcinoma of the oral cavity is reported. A 69-year-old man had black plaque on the oral floor and leukoplakia of the tongue, and the regional cervical lymph nodes were swollen. The patient underwent incisional biopsies of two lesion-associated regions in the oral cavity. The biopsies showed multiple primary cancers of malignant melanoma and squamous cell carcinoma. The patient underwent surgical resection of the lesions, bilateral neck dissection, and restoration with a pectoralis major musculocutaneous flap. Several cervical lymph nodes had metastases from malignant melanoma and squamous cell carcinoma. The patient received three cycles of DAV-Feron(dacarbazine, nimustine, vincristine, and interferon-beta) and three cycles of DTIC(dacarbazine) as adjuvant chemotherapy. After the therapy, new metastatic nodules were seen in the lung, bones, and skin. The patient died 11 months postoperatively.