A 77-year-old male patient with 2-vessel coronary artery disease and previous myocardial infarction underwent on-pump coronary artery bypass grafting (CABG). Following systemic heparinization, cardiopulmonary bypass using heparin coated circuit was started. Ten minutes after starting the cardiopulmonary bypass, the trans-oxygenerator pressure gradient rapidly increased accompanied by a rapid decrease of platelet counts. Emergency replacement of cardiopulmonary bypass circuit with a non-heparin coated one was performed because the development of heparin induced thrombocytepenia (HIT) was strongly suspected. On-pump CABG was accomplished as planned, and the postoperative course was uneventful. HIT might be ruled out as HIT specific antibodies were not detected in the intraoperative serum samples.