A hollow fiber membrane bundle configured to be used in an artificial lung and comprised of integrated hollow fiber membranes 31 has hollow portions through which a fluid passes. The hollow fiber membrane bundle is shaped as a cylinder body. In addition, the hollow fiber membrane 31 is tilted with respect to a central axis O of the cylinder body, is wound around the central axis O of the cylinder body, and satisfies the following conditions. An inner diameter φd1 of the hollow fiber membrane 31 is equal to or smaller than 150 µm, a tilt angle θ with respect to the central axis O of the cylinder body of the hollow fiber membrane 31 is equal to or smaller than 60°, and a ratio φD1/L of an outer diameter φD1 of the cylinder body to a length L of the cylinder body is equal to or greater than 0.4.
