A 56-year-old woman was admitted to our hospital because of cough and general fatigue. She was diagnosed with right-sided upper lobe lung cancer with multiple lung and brain metastases. Contrast-enhanced magnetic resonance imaging (MRI) of the brain showed cerebellar herniation. Her performance status (PS) deteriorated with nausea and disequilibrium caused by the brain metastases. An urgent craniotomy was performed, and pathological examination of the resected specimen revealed anaplastic lymphoma kinase (ALK)-rearranged pulmonary adenocarcinoma. After whole-brain radiotherapy (WBRT), administration of alectinib was initiated. The patient's PS improved rapidly from 3 to 1, and the multiple lung and brain metastases had decreased in size on computed tomography scan and contrast-enhanced MRI. This case suggests that it is justifiable to perform brain tumor resection when driver gene mutations are suspected in lung cancer patients with multiple brain metastases, even with a poor PS.