It has become possible in Japan to use high-dose metformin for patients with type 2 diabetes. The aim of this retrospective study was to determine the effects and safety of metformin in elderly patients with type 2 diabetes. The study subjects (98 patients who were treated with metformin) were assigned into two groups: (Ⅰ) 59 patients who were younger, aged less than 65 years, and (Ⅱ) 39 patients who were elderly, aged more than 65 years. The primary endpoint was the change in glycosylated hemoglobin A1c (HbA1c) at 12 weeks. The secondary endpoints were the safety variables, including hypoglycemic events and adverse events. Although HbA1c decreased significantly in both group Ⅰ (-0.5%) and group Ⅱ (-0.9%), the difference between the two groups in the change in HbA1c was not significant. There were no incidences of hypoglycemia or adverse events in either group. Metformin improved glycemic control in the elderly patients as well as in the non-elderly patients. It is necessary to examine what dose of metformin and serum creatinin level (Cre), eGFR is appropriate for elderly patients.