A 66-year-old woman was diagnosed with anaplastic lymphoma kinase (ALK)-rearranged lung adenocarcinoma, assigned a clinical stage of IIIA. She underwent chemoradiotherapy as the first-line treatment. She was admitted to our hospital one year later as she was experiencing severe headache, nausea, and vertigo. Head contrast-enhanced magnetic resonance imaging (MRI) revealed a total of 15 brain metastases. Although her performance status was 4, she was treated with alectinib. She refused to undergo whole-brain radiotherapy. Symptoms of brain metastases disappeared after starting alectinib therapy. A head contrast-enhanced MRI performed 13 days after starting alectinib therapy revealed that the number of brain metastases had markedly reduced. She was discharged from hospital 17 days after commencing alectinib therapy. In the above case study, alectinib therapy showed a beneficial effect on symptomatic brain metastases.