A flexible blood filter allowing smooth flow of a liquid and being excellent in separation resistance, pressure resistance and centrifugation resistance. The blood processing filter, includes a first filter element for removing aggregates from blood, a second filter element for removing leukocytes provided in the downstream of the first filter element, and a third filter element having gas permeability of 3 to 40 cc/cm2/sec per thickness of 1 cm provided between the second filter element and the outlet port. The filter contains a seal zone at a peripheral section which contains five layers including a layer of molten material and layers of molten material mixed with filter element material.