A 40-year-old man suffering from right cheek swelling was first diagnosed with ameloblastoma or anaplastic poorly-differentiated carcinoma of the head and neck region. He received 2 courses of CDDP/TXT chemotherapy (cisplatin 75 mg/m2, docetaxel 80 mg/m2) and achieved a partial response, but his carcinoma of the pancreas recurred. He also achieved a partial response with 2 courses of CBDCA/TXL regimen (carboplatin AUC=6, paclitaxel 200 mg/m2), but later died from his progressive disease. The autopsy revealed a pathological diagnosis of metastatic endocrine carcinoma of the pancreas. This case was close to a cancer with an unknown primary (CUP) site, and several favorable sub-sets of CUP have been identified, which are responsive to systemic chemotherapy. Poorly-differentiated neuroendocrine carcinomas like this case are highly sensitive to chemotherapy, and a careful pathological diagnosis may clarify its sensitivity to chemotherapy and the prognosis.