The use of opioid for treatment of cancer pain has become common with the spread of the WHO method for relief of cancer pain. However, it cannot yet be said whether such usage is appropriate. To understand how opioid analgesics are actually used, we conducted a nationwide mail-based questionnaire survey of doctors dealing with cancer pain treatment. Results indicate that at opioid rotation to fentanyl patch due to insufficient effect, the second opioid is used in a potency equianalgesic or less of the first, so the efficacy of the second is often insufficient. In cases where previous medication had an insufficient effect, taking rescue doses into account, we believe it is important to rotate opioid at 30- 50% above the equianalgesic dose.