We report a case of refractory cancer pain that was successfully treated with opioid switching by adding methadone to the preceding opioid. A 38-year-old man had severe epigastric pain and back pain because of paraaortic lymph node metastasis of a gastroesophageal junctional carcinoma. His pain was treated with continuous intravenous morphine administration and the frequent use of a rescue dose. When the morphine dose was increased, respiratory depression developed; thus, his pain was considered refractory to the morphine, and methadone was added on. The pain was relieved after initiating methadone, and the frequency of the rescue dose was markedly decreased. The methadone dose was gradually increased in parallel, and the morphine dose was reduced and finally discontinued. No methadone-induced side effects were noted, and the patient was discharged with good analgesia. In our case, adding methadone without decreasing the preceding opioid dose under strict monitoring made it possible to stably switch the opioid without increasing pain.