This invention relates to a blood processing filter comprising a sheet-like filter element, an inlet-side flexible container and an outlet-side flexible container that sandwich the filter element and are sealed thereto, an inlet port provided in the inlet-side flexible container for accepting blood before being processed by the filter element, and an outlet port provided in the outlet-side flexible container for discharging blood after being processed by the filter element. The blood processing filter also includes a flow channel securing sheet arranged between the filter element and the outlet-side flexible container. A flow channel hole through which blood processed by the filter element passes is formed in the flow channel securing sheet. The outlet port is provided so as to be capable of communicating with the flow channel hole.
