PURPOSE: To prevent adhesion of hollow yarn membranes to each other in the stage of bundling and to increase effective membrane area by providing an outwardly curved parts and inwardly curved parts in the cross sectional shape of the yarn perpendicular to the fiber axis of the hollow yarn membrane.

CONSTITUTION: A sprinning soln. contg. fiber-forming polymer is extruded from a spinning nozzle having an outwardly curved ejecting port and an inwardly curved ejecting port corresponding to the cross sectional shape 1 of an aimed hollow yarn membrane. The extruded hollow yarn having protruded parts is twisted at its plasticizing temp. to make the protruded parts to form spiral shape along the external surface of the hollow yarn. A hollow yarn type fluid separation device utilizing the thus obtd. hollow yarn membrane 2 as bundled body can prevent stagnation of fluid on the outside of the hollow yarn membrane, preventing also concentration polarization, channeling of the fluid, etc. Accordingly, the device is effectively useful for a blood dialyzer.