A 44-year-old woman with ALK-positive advanced adenocarcinoma of the lung was treated with crizotinib, and the lung lesions disappeared. The patient was treated with alectinib and chemotherapy, but brain metastases worsened; therefore, we performed an ALK resistance gene mutation test using plasma samples. Since no ALK resistance gene mutations were detected, we speculated that ALK inhibitors failed to achieve therapeutic effects due to poor transport to the central nervous system. Therefore, we switched to lorlatinib, and found a reduction in brain metastases. In ALK-positive advanced lung cancer, plasma-based resistance gene testing may be useful for treatment decisions.