An oxygenator which helps avoid bubbles in the blood from being discharged through the blood outlet port of the oxygenator includes a housing, a hollow fiber membrane bundle in the housing and formed by a multiplicity of hollow fiber membranes serving for gas exchange, gas-inlet and gas-outlet ports communicating with gas passages of the hollow fiber membranes, and a blood-inlet and blood-outlet ports. A filter member 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 projects from the housing and a passage enlargement 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 is allowed to reach the blood outlet port by being decelerated in the passage enlargement.