We report a 53-year-old man with cancer of an unknown primary site with an epidermal growth factor receptor mutation for which gefitinib was effective. In 2007, he complained of left gluteal pain and right cervical lymph node swelling. He was given a diagnosis of adenocarcinoma at the biopsy of the right cervical lymph node. Although metastases of multiple lymph nodes, bone, and bilateral adrenal glands were found, the primary site could not be determined on close examination, resulting in a diagnosis of cancer of unknown primary site(poor prognosis group). He was then treated with systemic chemotherapy. After he showed resistance to chemotherapy, he received gefitinib as third-line therapy because the tumor harbored an epidermal growth factor receptor(EGFR)mutation. Subsequently, multiple metastatic tumors gradually reduced and clinical benefit was observed for a long time.