A 57-year-old man initially presented with chief complaints of abdominal distension and anorexia. Positron emission tomography- computed tomography (PET-CT) scan showed ascites and multiple peritoneal metastases with abnormal uptake of fluorodeoxyglucose (FDG). The patient underwent endoscopy, biopsy, and cytology and was diagnosed with adenocarcinoma of unknown primary origin. He was treated with systematic chemotherapy, including carboplatin/paclitaxel (CBDCA/ PTX) and gemcitabine regimens. However, progressive disease (PD) complicated by intestinal obstruction was indicated. He was referred to our department for management. We performed surgery to resolve the intestinal obstruction and confirm the diagnosis. Appendix cancer was diagnosed intraoperatively. He was administered a modified fluorouracil plus Leucovorin and oxaliplatin(mFOLFOX6) /panitumumab regimen following surgery. The tumor had a good response to treatment, and the primary lesion was resected. After resection , the tumor was controlled by systemic chemotherapy for six months. However, the patient unfortunately died owing to arrhythmia. Most patients with cancer of unknown primary origin have a very poor prognosis because it is difficult to select appropriate treatment. Laparotomy can be effective in making a definitive diagnosis, as in the case described here.