A 50-year-old man was referred to our hospital because of a 2-month history of painful gingival swelling. Histopathological examination of the biopsy specimen showed a metastatic adenocarcinoma, and a chest-abdominopelvic CT showed multiple metastases in the lung, liver, and spleen, but failed to demonstrate the primary tumor. He had never complained of abdominal symptoms, and physical examination did not show any abnormality in the abdomen. However, immunohistochemical staining including caudal-related homeobox transcription factor (CDX-2) of the gingival tumor and PET-CT findings strongly suggested colorectal cancer as the origin. Colonoscopy then revealed a tumor in the rectum, and systemic chemotherapy was started immediately.