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 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; a bubble removal means comprising a filter member 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 bubble removal means is in contact with an outer surface of the hollow fiber membrane bundle and covers substantially all of the outer downstream surface of the hollow fiber membrane bundle, wherein the housing of the oxygenator is comprised of a housing body which is cylindrical or rectangular in cross-section and a passage enlargement is provided around the blood outlet port closer to the cylindrical or rectangular housing body around the upstream end of the blood outlet port, the passage enlargement is located downstream of the filter, characterized in that the bubble removal means further comprises a gas outlet hollow fiber membrane layer arranged upstream of the filter member, and the bubbles caught by the filter member are removed by the gas outlet hollow fiber membrane layer located upstream of the filter member. 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 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; a bubble removal means comprising a filter member 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 bubble removal means is in contact with an outer surface of the hollow fiber membrane bundle and covers substantially all of the outer downstream surface of the hollow fiber membrane bundle, wherein the housing of the oxygenator is comprised of a housing body which is cylindrical or rectangular in cross-section and a passage enlargement is provided around the blood outlet port closer to the cylindrical or rectangular housing body around the upstream end of the blood outlet port, the passage enlargement is located downstream of the filter, characterized in that the bubble removal means further comprises a gas outlet hollow fiber membrane layer arranged upstream of the filter member, and the bubbles caught by the filter member are removed by the gas outlet hollow fiber membrane layer located upstream of the filter member.