Inclusion Criteria:  Age 19-80 Cancer pain Admission for the control of cancer pain or consultation for the treatment of cancer pain Stable vital sign ECOG status ≤ 3 for more than 1 or 2 months Opioid-tolerant state No history of using sublingual fentanyl  Exclusion Criteria:  Noncancer pain Opioid naive baseline NRS pain score> 4 Current using sublingual fentanyl Difficult to assess cancer pain no evidence of disease(cancer) Planned surgical resection of cancer Allergy to fentanyl Severe renal and/or liver function Severe respiratory depression or uncontrolled COPD