A seventy‒year‒old man complaining of left arm weakness and gait disturbance was diagnosed with multiple tumors and severe edema in the brain as well as right lung mass lesion, suggesting brain metastases. He started whole brain radiation therapy but had to discontinue it since his neurological symptoms were worsened including paralysis, aphasia, and coma. These symptoms made it difficult to perform tumor biopsy for cancer diagnosis as well as oncogene mutations. Liquid biopsy, which examines EGFR gene mutations in plasma sample, revealed EGFR L858R point mutation. Treatment with osimertinib improved his symptoms, resulting in discharge to home. Even a patient severely ill with metastatic brain tumors can benefit from the molecular‒targeted therapy using liquid biopsy to diagnose EGFR‒mutated lung cancer, suggesting an important differential diagnosis in such patients.