Introduction: We report a case of severe headache caused by lung cancer metastasis to the base of the skull that was difficult to diagnose due to a lack of imaging evidence. Case: A 70-year-old man diagnosed with advanced lung cancer experienced sudden, severe headache. He was diagnosed as having a tension-type headache because magnetic resonance imaging of his head failed to detect any pathology. He was prescribed various drugs, which except for strong opioids failed to treat his headache. He referred to our palliative care unit to treat the pain. Re-evaluation of his head CT revealed metastasis to the clivas. His pain was treated with rapid titration of subcutaneous oxycodone injection. Conclusion: Even if radiographic investigations fail to identify the metastasis, the patient should be re-evaluated if the headache worsens and/or is accompanied with cranial nerve dysfunction.