Systemic administration of opioids rarely causes pruritus, although its mechanism is still not clearly understood. We report an intractable pruritus induced by a change in the dosage form of opioids with the same dose by the conversion ratio, which promptly disapeared with opioid switching. A 80-year-old female experienced worsening dorsal pain and abdominal pain due to recurrent pancreatic cancer. The relief of pain was insufficient with the administration of oral hydromorphone 10 mg/day, changing to continuous intravenous hydromorphone 3 mg/day considering adjustability. The next day, her pain was reduced, but the pruritus appeared. Administering oral and topical antihistamines was ineffective. Her pruritus tended to worsen with continued administration of continuous intravenous hydromorphone at the same dose. Hence we switched to fentanyl patch 0.6 mg/day. The following day, her pruritus significantly decreased, and two days later, her pruritus almost disappeared. This case suggests that opioids switching to fentanyl patch may be effective to relieve pruritus caused by hydromorphone.