Neuroendocrine carcinomas occur mainly in the lung and digestive tract and rarely arise in the oral cavity. We report a case of neuroendocrine carcinoma arising in the floor of the mouth. A 77-year-old man was referred to our department because of contact pain in the left side floor of the mouth in April 2013. We performed a biopsy and diagnosed a carcinoma of floor of the mouth (suspected to be neuroendocrine carcinoma). Tumor resection, split-thickness skin grafting, and bilateral submandibular duct relocation were performed in May 2013. Histologically, the tumor was composed of polygonal cells. The tumor cells showed nuclei with dark-stained-chromatin and slightly rosette-like growth patterns in the subepithelium. Immunohistochemically, the tumor cells were positive for CD56, chromogranin, and TTF-1, partially positive for CD99, and negative for synaptophysin, CK20, and CD117. The mitotic rate of the tumor cells was 22/10HPFs, and the Ki-67 index exceeded 90%. The pathological diagnosis was neuroendocrine carcinoma. Ten months after operation, left cervical lymph node metastases and the right iliac metastases were found, and the patient died of multiple organ metastases after 11 months.