An oxygenator comprising: a housing having an interior; a blood inlet port in the housing through which blood is adapted to flow, the blood inlet port opening to outside the housing and communicating with the interior of the housing to introduce the blood into the interior of the housing; a hollow fiber membrane bundle positioned in the interior of the housing and comprised of a multiplicity of integrated hollow fiber membranes configured to subject the blood introduced into the housing to gas exchange; the hollow fiber membranes each possessing a lumen extending between opposite ends of the hollow fiber membrane forming a gas passage for passage of gas; a gas inlet port in the housing through which gas is adapted to flow, the gas inlet port opening to outside the housing and communicating with the gas passages of the hollow fiber membranes to introduce the gas into the gas passages a gas outlet port in the housing through which gas is adapted to flow, the gas outlet port opening to outside the housing and communicating with the gas passages of the hollow fiber membranes to discharge the gas in the gas passages; a blood outlet port in the housing through which blood which has been subjected to the gas exchange is adapted to flow, the blood outlet port opening to outside the housing and communicating with the interior of the housing to discharge from the housing the blood which has been subjected to the gas exchange; characterized in that a filter member is positioned at a side of the hollow fiber membrane bundle closer to the blood outlet port and constructed to catch bubbles in the blood which has been subjected to the gas exchange, wherein the filter member (41 A) is in contact with an outer surface of the hollow fiber membrane bundle (3 A) and covers substantially all of the outer downstream surface of the hollow fiber membrane bundle, the oxygenator possesses a nearly cylindrical form in its entire or overall exterior shape and the oxygenator possesses a passage enlargement (281, 282) which is provided in a region nearby the blood outlet port and close to the cylindrical housing body or housing in a base region of the blood outlet port which has a lumen in communication with a lumen of the passage enlargement (281, 282) which is provided as a region where the passage is increased in its cross-sectional area. a housing having an interior; a blood inlet port in the housing through which blood is adapted to flow, the blood inlet port opening to outside the housing and communicating with the interior of the housing to introduce the blood into the interior of the housing; a hollow fiber membrane bundle positioned in the interior of the housing and comprised of a multiplicity of integrated hollow fiber membranes configured to subject the blood introduced into the housing to gas exchange; the hollow fiber membranes each possessing a lumen extending between opposite ends of the hollow fiber membrane forming a gas passage for passage of gas; a gas inlet port in the housing through which gas is adapted to flow, the gas inlet port opening to outside the housing and communicating with the gas passages of the hollow fiber membranes to introduce the gas into the gas passages a gas outlet port in the housing through which gas is adapted to flow, the gas outlet port opening to outside the housing and communicating with the gas passages of the hollow fiber membranes to discharge the gas in the gas passages; a blood outlet port in the housing through which blood which has been subjected to the gas exchange is adapted to flow, the blood outlet port opening to outside the housing and communicating with the interior of the housing to discharge from the housing the blood which has been subjected to the gas exchange; characterized in that a filter member is positioned at a side of the hollow fiber membrane bundle closer to the blood outlet port and constructed to catch bubbles in the blood which has been subjected to the gas exchange, wherein the filter member (41 A) is in contact with an outer surface of the hollow fiber membrane bundle (3 A) and covers substantially all of the outer downstream surface of the hollow fiber membrane bundle, the oxygenator possesses a nearly cylindrical form in its entire or overall exterior shape and the oxygenator possesses a passage enlargement (281, 282) which is provided in a region nearby the blood outlet port and close to the cylindrical housing body or housing in a base region of the blood outlet port which has a lumen in communication with a lumen of the passage enlargement (281, 282) which is provided as a region where the passage is increased in its cross-sectional area.