PURPOSE: To allow oxygen-containing gas flowing-in from an inlet to flow to an interior, in a hollow yarn membrane type oxygenator having specific fine pores, by providing a gas guide part to a hollow yarn membrane bundle so as to extend in the internal direction of said bundle from the outer periphery thereof or in the vicinity thereof and making the filling density of the hollow yarn membranes more coarse than that of other part.

CONSTITUTION: Each of the hollow yarn membranes 2 used in a hollow yarn membrane type oxygenator 1 is porous and has large number of pores piercing through the wall thereof. As a gas exchange membrane, one having a thickness of 5W80μm, a void ratio of 20W80%, a pore size of 0.01W5μm and an inner diameter of 100W1,000μm is pref. In the hollow yarn membrane bundle 4 in the vicinity of the gas inflow port 13 of an oxygenator 1, a gas guide part 3 extending in the internal direction of the hollow yarn membrane bundle 4 from the outer periphery thereof or the vicinity thereof is provided to form a part where the filling density of the hollow yarn membranes 2 is lower than that of other part. The gas guide part 3 forms a part where the hollow membrane yarns 2 are not substantially present and is provided in order to allow the oxygen-containing gas flowing-in from a gas inflow port 13 to certainly flow to the center part of the hollow yarn membrane bundle 4.