Thrombocytopenia is often caused by myelosuppression during chemotherapy. However, when platelet transfusions are required, pathological conditions such as idiopathic thrombocytopenic purpura(ITP)and thrombotic thrombocytopenic purpura( TTP)also occur. We report a case of Merkel cell carcinoma complicated with severe thrombocytopenia treated with carboplatin/etoposide regimen after surgery. The patient's platelet count did not increase in spite of platelet transfusions. However, the platelet count increased after steroid treatment was chosen under the diagnosis of ITP. Subsequent examinations revealed that the patient had HLA antibody, which caused the platelet transfusion refractoriness. When the platelet count does not increase in spite of platelet transfusions during chemotherapy, the possibility that the platelet transfusion refractoriness is due to the presence of HLA antibody should be considered.