Inclusion Criteria:

- Estrogen receptor positive breast cancer

- Clinically measurable tumor size(stage II/III)

- No evidence of distant metastasis

- Postmenopause women 'Age≥60yr' OR 'previous bilateral oophorectomy' OR 'FSH>30 with
no bleeding history within 1yr'

- ECOG 0-2

- Adequate hepatorenal, bone marrow function 'serum Cr<1.4mg/dL' AND 'Bilirubin< upper
limit of normal x 1.5 AND 'AST/ALT < upper limit of normal x 1.8 AND 'ALP < upper
limit of normal x 1.8 AND 'Hemoglobin >10 g/dL' AND 'ANC >1,500/mm3' AND 'Platelet
>100,000/mm3'

- Spontaneous signed into the written informed consent

Exclusion Criteria:

- Who does not meet the above inclusion criteria

- History of other carcinoma

- Uncontrolled infection

- History of psychiatric, epileptic disease

- Male breast cancer

- Diabetes 'HbA1c≥6.5' OR 'FBS≥126mg/dL'

- Hypersensitivity or intolerance to metformin

- Risk of metformin associated lactic acidosis 'Congestive heart disease of NYHA Class
llll/IV' OR 'History of other acidosis' OR 'Alcohol intake more than either 3 bottles
beer OR 1 bottle Soju'

- During medication of metformin, sulfonylureas, thiazolidinediones, insulin

- Diffuse microcalcification in mammogram

- Multiple OR bilateral OR inflammatory breast cancer

- Chemotherapy or endocrine therapy within 2yr due to history of breast cancer