This is the clinical trial designed to assess the cardiac diastolic function of long term treatment with teneligliptin compared to that without teneligliptin in patients with type 2 diabetes mellitus by two arms; one includes patients showing E/e' by echocardiography less than 8, the other includes patients showing E/e' by echocardiography more than 8.

Inclusion Criteria:

• Asians aged from 20 to 85 years old at baseline
• Patients with type 2 diabetes mellitus and including either A) or B) criteria.

A) Patients necessary to start the treatment using anti-diabetic agent(s) or to change the anti-diabetic agent(s).

B) Patients possible to change the anti-diabetic agent(s).

• Patients with left ventricular ejection fraction more than 40%
• Patients with written informed consent

Exclusion Criteria:

• Patients with type 1 diabetes mellitus
• Patients with slowly progressive type 1 diabetes mellitus positive for pancreatic islets related autoimmune antibody as GAD antibody or IA-2 antibody or ICA antibody
• Patients with diabetes mellitus caused by evident genetic factors
• Patients with diabetes mellitus caused by secondary factors as endocrine disease or liver disease
• Patients with diabetic ketoacidosis or hyperosmolar hyperglycemic syndrome
• Patients with any severe infectious diseases or planed any surgical treatments or suffered any severe traumas
• Patients with severe liver dysfunction
• Patients with hypophyseoprivic or adrenal insufficiency
• Patients under malnutrition or starved state or irregular caloric intake or calorie insufficiency or hyposthenia
• Patients judged to be unsuitable for the study as they are planning to exercise intensively
• Patients judged to be unsuitable for the study as they may drink excessively or abuse drugs
• Patients showing QT prolongation in the electrocardiogram
• Patients with any past histories of heart failure showing NYHA classification grade more than 3 at baseline
• Patients with any past histories of acute coronary syndrome or coronary intervention or cardiac surgery developed within 6 months
• Patients with any surgical past histories of mitral valve replacement or mitral valve repair or severe calcification of mitral valve
• Patients already treated with Teneligliptin
• Women with breast-feeding
• Pregnant women or patients who have possibilities of pregnancy
• Patients expected to live less than 3 years
• Patients with any past histories of drug hypersensitivity against Teneligliptin
• Patients already involved in any other interventional clinical trials or planned to be involved
• Patients judged to be inappropriate for the study by the doctors in charge