Aims/IntroductionInsulin degludec, a new long‐acting insulin analog, showed its better glycemic control and reduced risk of hypoglycemia. This is the first prospective observational study that evaluated the efficacy and safety of insulin degludec in routine clinical practice.Materials and MethodsJapanese patients with type 1 or type 2 diabetes mellitus receiving basal–bolus insulin therapy were switched their basal insulin to degludec, and prospectively observed over a 24‐week course. The Diabetes Therapy‐Related Quality of Life questionnaire was administered before and 12 weeks after switching.ResultsThe participants were 80 diabetes patients = (type 1, 44; type 2, 36). In the type 1 group, there was no difference in glycated hemoglobin levels between the pre‐switching and 24‐week measurements (from 62 to 62 mmol/mol, P = 0.768), whereas the daily insulin dose (per kg of bodyweight) decreased significantly (basal, from 0.25 to 0.20 U/kg, P < 0.001; bolus, from 0.40 to 0.37 U/kg, P = 0.001). In the type 2 group, glycated hemoglobin levels decreased after switching (from 60 to 58 mmol/mol, P = 0.028). In the type 1 group, the frequency of hypoglycemia decreased significantly after switching, but not significantly in the type 2 group. Patient satisfaction with the control of hypoglycemia tended to improve in the type 1 group.ConclusionsCompared with existing long‐acting insulin, degludec can maintain glycemic control at a lower insulin dose and frequency of hypoglycemia in type 1 diabetes, while it can improve glycemic control at an equal insulin dose in type 2 diabetes.