Background: We experienced two cases where fentanyl relieved pregnant women of their cancer pain. Case 1: A pain in the lower back of a 30-year-old woman since the 30th week of her pregnancy for multiple myeloma. We started fentanyl continuous intravenous infusion on the first day of the 34th week and gradually increased the dosage to 32 μg/h. Elective cesarean section was performed on the 1st day of week 36. The child was born without problems. Case 2: A 34-year-old woman was diagnosed with stomach cancer in the 22nd week of her pregnancy. Fracture on the twelfth thoracic vertebra was also found. Continuous-subcutaneous-infusion of fentanyl was started and the dosage was escalated to 24 μg/h , which reduced her pain. Chemotherapy was effective for her cancer and did not affect the fetus. At week 34, elective cesarean section was carried out. Because the child presented cyanosis immediately after birth, tracheal intubation was performed, but extubated on the next day without problem. Conclusion: Fentanyl was found to be effective for reducing cancer pain in the two cases. Although the existence of causal relationship is unknown, cyanosis was observed in one case.