A hollow fiber membrane type oxygenator is provided wherein a bundle of a plurality of hollow porous fibers presenting gas exchange membranes axially extends through a housing to define a first flow path for oxygen, a pair of partitions support the opposed open ends of the hollow fibers to isolate the open ends from the first flow path to define a second flow path through the fiber cavities for blood, and the micropores in the hollow fibers are filled with a silicone compound, for exmaple, a RTV silicone rubber or a blend thereof with a silicone oil while the hollow fibers on their side wall are substantially free of a layer of the silicone compound. The oxygenator is manufactured by prefabricating an oxygenator module of the above-mentioned construction, impregnating the hollow fibers with a solution of the silicone compound, and passing a cleaning liquid to remove excessive silicone compound, thereby filling only the micropores with the silicone compound.