An oxygenator (1) which helps avoid bubbles in the blood from being discharged through the blood outlet port (28) of the oxygenator (1) includes a housing (2), a hollow fiber membrane bundle (3) in the housing (2) and formed by a multiplicity of hollow fiber membranes (31) serving for gas exchange, gas-inlet (26) and gas-outlet (27) ports communicating with gas passages (32) of the hollow fiber membranes, and a blood-inlet (51) and blood-outlet ports (28). A filter member (4) is provided on a side closer to the blood outlet port of the hollow fiber membrane bundle and serves to catch bubbles in blood. The blood outlet port (28) projects from the housing and a passage enlargement (281) is provided in a vicinity of the end of the blood outlet port closer to the housing and having an increased passage cross-sectional area. The blood passed the filter member (4) is allowed to reach the blood outlet port (28) by being decelerated in the passage enlargement.
