In this study, to evaluate the efficacy of insulin degludec as a basal insulin for basal-bolus regimen for Japanese patients with type 1 diabetes mellitus who are treated with twice-daily basal insulin injection therapy, we investigated glycemic control, daily, and day-to-day variability in plasma glucose using continuous glucose monitoring before and after switching to once-daily insulin degludec injection in 22 patients with type 1 diabetes. Here, INS is linked to diabetes mellitus.