Neck mass is a common head and neck problem observed in patients visiting local clinics, and it can be caused by bacterial or viral infections, cancer, or other causes. Structures within the neck include the lymph nodes, thyroid gland, parathyroid glands, and salivary glands. We present a case report of a patient who had a left neck mass for several months. The patient was diagnosed with a metastatic malignant tumor after a physical examination, imaging examination, endoscopic examination, fine needle aspiration biopsy, and direct biopsy of the mass; however, the primary tumor site was uncertain. Although nasopharyngeal carcinoma was highly suspected during the imaging examination, the endoscopic examination did not confirm the diagnosis. An Epstein-Barr virus serological examination also revealed negative results; therefore, the primary lesion site remained unknown.