The present invention relates to a material for aggregating blood cells, used in the preparation of a paraffin block for diagnosing circulating tumor cells (CTCs), and a method for preparing a paraffin block using the same, and more specifically, to a method for detecting CTCs present in the blood through blood sample collection in the preparation of a paraffin block from CTCs, which are present in a very small amount in the blood. The present invention is non-invasive and simple, and thus is very useful for the diagnosis of cancer recurrence and metastatic cancer and for prognosis prediction, and can be a remarkable breakthrough in cancer treatment through the analysis of the onset, metastasis and recurrence mechanisms of cancer.