Background Diabetes mellitus (DM) has been more prevalent. American Diabetes Association (ADA) proposed the Standards of Medical Care in Diabetes-2022. For nutritional therapy, low carbohydrate diet (LCD) has been recognized for its benefits. Authors have continued diabetic research concerning LCD and meal tolerance test (MTT). Case Presentation The case is 61-year-old male with type 2 diabetes mellitus (T2DM) for years. His hemoglobin A1c (HbA1c) increased to 7.8% in autumn 2021, and further evaluation and treatment was conducted including LCD, daily check of meal and carbohydrate amount, 75 g OGTT, glucagon stimulation test (GST) and others. Results He was on super-LCD method including 12% of carbohydrate. His carbohydrate intake amount and 45-minutes post-prandial blood glucose showed significant correlation. The results of 75 g OGTT twice in May 2020 and December 2021 showed that similar pattern of glucose and insulin responses and insulinogenic index (IGI). In contrast, they showed decreased fasting immuno-reactive insulin (IRI) and Homeostasis model assessment insulin resistance (HOMA-R). For GST, C-peptide showed normal response. Discussion and Conclusion Judging from the results of MTT, OGTT, GST and IGI, he seems to show rather decreased insulin resistance by LCD associated with preserved insulin secretion ability to some degree. Further investigation would be required from pathophysiological point of view.