PURPOSE: To obtain enough oxygen adding ability even when a film area is decreased by obtaining special relation in the length of a part which is specified by an internal wall and the effective length of a hollow yarn flux.

CONSTITUTION: For the length l1 of the internal wall specifying part in a housing 1 and a hollow yarn flux effective length l2, the relation of l1≥l2/4 is obtained. The hollow yarn flux effective length l2 is the mean value of a maximum length la and a minimum length of lb of a hollow yarm flux 11 along the central shaft direction of the housing 1 between upper and lower partition walls 6. When the length l1 of the internal wall specifying part is set to be more than 1/4 of the hollow yarn flux effective length l2, blood to execute channeling (short pass) on the internal wall surface of the housing 1 can be reduced and large oxygen moving quantity can be obtained with the small film area. Namely, the blood made flow from a blood flowing-in port 8 is evenly made flow into the hollow yarn flux 11 and uniformly distributes between porous hollow yarn films 5. Then, oxygen is added and and as a result, the high oxygen adding ability can be obtained.