Study subjects will be randomly assigned to the three groups and receive the study drug for maximum of 156 weeks and undergo blood samplings and other diabetes mellitus-related tests. The aim of the present study is to evaluate the durability of glycemic control over 3 years for patients with type 2 diabetes on diet and exercise therapy treated with oral hypoglycemic drug monotherapy.

Inclusion Criteria:

1. Patients diagnosed with type 2 diabetes mellitus who are 20 years of age or older at the time of obtaining consent.
2. Patients being treated with diet and exercise therapy only at the time of eligibility test However, if the patient is taking one oral hypoglycemic drug at the time of obtaining consent, the patient must be able to wash out the oral hypoglycemic drug for at least 12 weeks before the start of study treatment.
3. Patients whose HbA1c level is between 7.0% and 9.0% as measured at the time of the eligibility test.
4. Patients who have given written consent to participate in this study.

Exclusion Criteria:

When consent is obtained

1. Patients with type 1 diabetes mellitus
2. Patients who have been given more than 2 oral hypoglycemic drugs within 12 weeks
3. Patients who have received glucagon like peptide-1 receptor agonist (short-term use of insulin for trauma or educational admission) within 1 year or less
4. Patients with proliferative retinopathy (except for patients with stable treated proliferative retinopathy)
5. Patients with severe diabetic neuropathy (patients with severe symptoms and significant support for daily life)
6. Patients with a contraindication to Imeglimin, Metformin, or Vildagliptin
7. Patients with severe obesity (BMI 35 kg/m^2 or more)
8. Patients with NYHA (New York Heart Association) cardiac function classification of Grade III or IV within 1 year of evaluation
9. Excessive regular drinkers
10. Patients with a previous history of lactic acidosis
11. Patients with severe cachexia, diabetic coma or precoma
12. Patients with severe infections, surgical patients and those with serious injuries
13. Patients who are pregnant, who are planning to be pregnant, or who are breastfeeding
14. Patients who are undergoing treatment for malignancy or those with a history of treatment for malignancy within 5 years
15. Patients who are participating in a clinical study with other interventions
16. Patients to whom a responsible physician/investigator judged inappropriate for participating in the study In case of eligibility testing
17. Patients with an estimated glomerular filtration rate(eGFR) of 45 mL/min/1.73m^2 or less including those undergoing dialysis
18. Patients with severe hepatic disorders (Child-Pugh classification Grade C)