We recently reported that the administration of miglitol alone just before breakfast improved postprandial hyperglycemia and increased active glucagon-like peptide-1 (GLP-1) levels after lunch in men without diabetes. Miglitol and dipeptidyl peptidase-4 inhibitors, such as sitagliptin, enhance plasma active GLP-1 concentrations via different mechanisms; therefore, combined therapy with these agents was more effective than monotherapy. In this study, we compared the effectiveness of the administration of miglitol alone just before breakfast on the plasma glucose, serum insulin and glucagon, and plasma incretin levels in sitagliptin-treated patients with type 2 diabetes. We measured the plasma glucose, serum insulin and glucagon, plasma active GLP-1, and total glucose-dependent insulinotropic polypeptide levels before breakfast, at 120 min after breakfast, before lunch, and 60 and 120 min after lunch in patients with diabetes who are receiving sitagliptin. This trial was performed for the following 2 days on each subject (Day 1: no miglitol, Day 2: miglitol alone [50 mg] administered just before breakfast). The area under the curve (AUC) of the plasma glucose levels after lunch in the miglitol-treated group tended to be lower than that in the miglitol-untreated group, but the difference was not statistically significant. Miglitol alone administered at breakfast increased the AUC of the active plasma GLP-1 levels after lunch in sitagliptin-treated patients with diabetes. Our results suggest that the once-daily administration of miglitol as a "GLP-1 enhancer" in combination with sitagliptin was effective for the treatment for patients with diabetes.