Inclusion Criteria:  1. Patients over 20 years of age undergoing off-pump coronary artery bypass graft  Exclusion Criteria:  Emergency operation Minimal invasive coronary artery bypass graft Cooperative operation with other surgery Patients who need a mechanical ventricular assist device Patients with chronic kidney disease (eGFR < 30) Patients who have a end stage renal disease and need a hemodialysis Patients with a low hemoglobin (Hb < 10) before surgery Patients who have a some disease(leukemia, myeloma, aplastic anemia, sickle cell anemia, etc) affecting hematopoiesis Patients with a liver cirrhosis Patients who receive vit. D, iron supplements, erythropoietin within 6 months Patients who cannot understand the informed consent (eg. Foreigner, patients with cognitive dysfunction, etc) Patients who disagree this study