The present study used intermittently scanned continuous glucose monitoring (isCGM) in 10 patients with type 1 diabetes mellitus (T1DM) to evaluate the efficacy and safety of 7-day outpatient treatment with the combination of intensive insulin therapy and sodium-glucose transporter 2 inhibitor (SGLT2-I). All participants wore isCGM and were treated with either 50 mg/day ipragliflozin or 5 mg/day dapagliflozin. The primary outcome, percent time with glucose at 70–180 mg/dL (TIR: time in range), improved significantly following the addition of SGLT2-I (p = 0.005). TIR increased from 36.0% before addition of SGLT2-I to 70.7% on day 7. Although none of the patients achieved TIR of 70% or higher before the addition of SGLT2-I, 6 patients met that criteria TIR on day 7. The secondary outcome measures, standard deviation (SD) of glucose, average plasma glucose, percent time with glucose at >180 mg/dL (TAR: time above range), maximum plasma glucose, high blood glucose index (HBGI) and average nocturnal plasma glucose (midnight to 05:59 AM) detected by isCGM, also improved significantly by SGLT2-I. There were no significant differences in percent time with glucose at <70 mg/dL (TBR: time below range), minimum plasma glucose and low blood glucose index (LBGI). Our results using isCGM in an actual clinical setting showed that 7-day use of SGLT2-I with intensive insulin therapy improved plasma glucose fluctuations and mean plasma glucose levels without inducing hypoglycemia in patients with T1DM.