Combination therapy with an α-glucosidase inhibitor and insulin is commonly performed for type 2 diabetes mellitus. We conducted a placebo-controlled double-blind comparative study to investigate the efficacy of combination therapy with miglitol and insulin. The patients with T2DM on insulin therapy were randomly assigned to either a miglitol treatment group (Group M) or a placebo group (Group P) and treated for 12 weeks. Meal tolerance tests were conducted at the observation period, week 0 and week 12 of the treatment period. Mean values of decrease in 1-h- and 2-h postprandial plasma glucose level were significantly larger in Group M than in Group P (60.3 ± 70.1 mg/dl vs. -5.1 ± 68.2 mg/dl (P < 0.001)), as was HbA1c as well (0.36 ± 0.66% vs. -0.03 ± 0.56% (P < 0.001)). Adverse events included abdominal distension and flatulence, which were significantly more frequent in Group M. The frequency of nocturnal hypoglycemia events tended to be reduced in Group M. Combined use of insulin and miglitol is useful for postprandial glucose regulation and improves glycemic control.