Objective We performed oral glucose tolerance testing (OGTT) with 75 g of glucose in patients with HbA1c 5.6~6.4% and investigated blood glucose and insulin secretory functions. The objective was to investigate impaired glucose tolerance and pancreatic β-cell function in patients with HbA1c < 6.5% during regular health checkups. Methods 75 g OGTT were performed to patients with HbA1c 5.6-6.4% at regular health checkups or outpatient examinations:214 (75 males, 139 females).Mean age 65.2 years. Blood glucose and pancreatic β-cell function were investigated. Pancreatic β-cell function was evaluated in terms of the disposition index, calculated as Insulinogenic Index (I.I.) /Fasting　IRI (F.I.) and insulin sensitivity from OGTT (ISOGTT) x aggregate insulin at the time of OGTT (before, 30 min, 60 min)/aggregate blood glucose (befor, 30min, 60min) Results Blood glucose values in 75-g OGTT were classified as diabetic in 35 subjects (16%), borderline in 122 (57%), and healthy in 57 (27%). 73% of subjects exhibited impaired glucose tolerance. Mean HbA1c values for the different categories by blood glucose in 75-g OGTT were Diabetic/Borderline/ Healthy 5.97%/5.95%/5.80%. 62.1% of subjects with HbA1c 5.6~5.9% and 88.9% of subjects with HbA1c 6.0~6.4% were exhibited impaired glucose tolerance. I.I. /F.I. were Diabetic/ Borderline/ Healthy0.05/0.08/0.24. ISOGTT × AUC (ins/glu) were Diabetic/Borderline/ Healthy0.79/1.20/2.10. Diabetic & Borderline were significantly lower values from healthy . Conclusion Many subjects with HbA1c 5.6-6.4% exhibited insulin resistance and decreased pancreatic β-cell function. It may be necessary to perform OGTT and provide lifestyle guidance if required. 250word.