The invention provides a hollow fiber membrane module and a manufacturing method thereof. In the hollow fiber membrane having a gradient structure having a larger diameter from the inner surface side toward the outer surface, even when blood flows at a small flow rate, it is possible to prevent the blood from staying while flowing from the head interior into the hollow fiber membrane, thereby suppressing the attachment of blood cell components to a hollow fiber membrane inlet. The hollow fiber membrane module includes a cylindrical container, a bundle of hollow fiber membranes (3), a perfusion encapsulating resin portion, and a head portion, wherein the hollow fiber membrane (3) has gradient structure having a larger diameter from the inner surface side toward the outer surface. An inclined surface (10) inclined by 10 to 70 degrees with respect to an opening end face (3a) is formed on the entire circumference of the open end of the hollow fiber membrane (3), and the inclined surface (10) is formed by extending from the outer end (3) in the membrane thickness direction circumferentially toward the inside.